Method for producing rare earth bond magnet, composition for rare earth bond magnet, and rare earth bond magnet

ABSTRACT

A method of the invention for manufacturing a rare earth bonded magnet has the steps of mixing a rare earth magnet powder, binder resin and an additive at a predetermined ratio, kneading the mixture at a temperature not lower than the thermal deformation temperature of the binder resin, granulating or graining the kneaded blend to granules of an average size ranging from 0.01 mm to 2 mm or so, conducting a compacting molding of the granulated material at a first temperature at which the binder resin is softened or molten, and cooling the molded body while keeping the molded body under pressure at least over a period in which the molded body cools down to a second temperature which s below the first temperature, whereby a rare earth bonded magnet is obtained having a low porosity, high dimensional precision and superior magnetic characteristic.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a rare earthbonded magnet by binding magnet powder containing rare earth elements bya binder, a composition for use in the manufacture of a rare earthbonded magnet, and to a rare earth bonded magnet.

BACKGROUND ART

Bonded magnets are manufactured from mixtures (compounds) of magneticpowders and binding resins (organic binders), by pressure-molding themixtures into desired magnet shapes. Among such bonded magnets, aspecial type of bonded magnet referred to as a rare earth bonded magnetis made up from a magnet powder which is composed of a magnetic materialcontaining a rare earth element or elements. Methods of manufacturingrare earth bonded magnets are disclosed, for example, in Japanese PatentPublication No. 53-34640, Japanese Patent Publication No. 46-31841,Japanese Patent Publication No. 04-74421, Japanese Patent Laid-Open No.59-136907, Japanese Patent Laid-Open No. 59-213104, Japanese PatentLaid-Open No. 02-153509, Japanese Patent Laid-Open No. 60-211908,Japanese Patent Laid-Open No. 60-216523, Japanese Patent Laid-Open No.61-164215, Japanese Patent Laid-Open No. 59-103309 and Japanese PatentLaid-Open No. 03-108301.

The methods of manufacturing rare earth bonded magnets are broadlysorted into compaction molding, injection molding and extrusion molding.

In compaction molding, the aforesaid compound is packed in a press moldand compacted at a room temperature so as to form a green body.Subsequently, when the binding resin is a thermosetting resin, the resinis hardened, whereby a magnet is obtained. This method enables themolding to be carried out with smaller amount of binding resin thanother methods, resulting in a smaller resin content in the productmagnet, thus advantageously contributing to improvement in the magneticcharacteristics of the magnet.

Extrusion molding is a method in which heated molten compound extrudedfrom an extruder die is solidified by cooling and then cut at a desiredlength, whereby a magnet is obtained. This method in one hand offers anadvantage in that it permits easy production of thin-walled or elongatedmagnet by virtue of a comparatively large molding versatility on theshape of product magnet, but on the other hand suffers from a problem inthat it requires, in order to ensure a sufficiently high fluidity of themolten compound during the molding, a greater amount of binding resin tobe used as compared with the compaction molding method, with the resultthat the magnetic characteristics are impaired due to increased resincontent in the product magnet.

In injection molding, the aforesaid compound, which has been heated andmolten to exhibit sufficiently high fluidity, is poured into a mold soas to form a magnet of a desired shape. This method offers moldingversatility on the magnet shape even greater than that offered by theextrusion molding method, enabling easy fabrication of magnets havingirregular configurations. However, this method requires higher level offluidity of the molten compound and, hence, a greater content of thebinder resin than required in the extrusion molding method, resulting inpoor magnetic characteristics of the product magnet due to increasedcontent of the binder resin in the product magnet.

Among these methods, the compaction molding method enables production ofmagnets having superior magnetic performance as compared with othermethods. Manufacture of bonded magnets by the known compaction moldingmethod, however, suffers from the following disadvantages.

Firstly, it is to be pointed out that rare earth bonded magnetsmanufactured by this method tend to exhibit high porosity, which reducesmechanical strength and corrosion resistance of the product magnet.Hitherto, therefore, countermeasures have been taken in the compactionmolding, such as use of high-pressure molding technique which employs acompaction pressure as high as 70 kgf/mm² and anti-corrosion coating onthe molded product. Elevated compaction pressure, however, heavilyburdens the mold and molding machine, which in turn requires a greaterdimensions of the mold and molding machine, incurring a rise of theproduction costs. In addition, the anti-corrosion coating does notachieve sufficient improvement in the resistance to corrosion.

A second problem is as follows. The compound is pelletized beforesubjected to the molding. It is often difficult, however, to smoothlycharge the pellets of the compound into the mold and to completely fillup the mold cavity. In addition, pelletized compound does not permitdelicate control of the rate of supply of the compound into the mold.

A third problem is as follows. When compaction molding is conducted on acompound containing a thermosetting resin, the compaction is effected atroom temperature, regardless of whether the thermosetting resin is ofthe type which is in solid phase at the room temperature or of the typewhich is in liquid phase at the room temperature. Therefore, when theformer type of thermosetting resin, i.e., the solid-phase resin, isused, moldability of the material is impaired, tending to exhibitgreater porosity than that obtained when a thermoplastic resin is used.In addition, mechanical strength also tends to be reduced due toinferior dispersibility of the resin and the magnet powder. When thelater-mentioned resin, i.e., the liquid-phase resin, is used, physicalproperties of the resin tends to be sensitively varied in accordancewith the molding environment, e.g., temperature and humidity, oftenresulting in inferior charging of the mold, although a green body ofhigh density is obtainable.

The second and third problems mentioned above cause the dimensions ofthe product magnets to substantially fluctuate from the targetdimensions. Namely, the dimensional precision is impaired and themolding cannot be conducted at high degree of stability. Thesedeficiencies are serious particularly when small-sized magnets are to bemanufactured.

In order to obtain a product magnet in conformity with the targetdimensions despite the substantial fluctuation in the dimensions, it isnecessary that the molded article has dimensions greater than the targetdimensions and that such molded article is subjected to a secondaryprocessing such as milling or grinding into final shape and dimensions.Such a secondary work increases the number of steps of the manufacturingprocess, and increases the risk of production of unacceptable products,with the results that the production efficiency is lowered and the costof production is raised.

The present inventors have discovered that one of the causes of thefirst to third problems described above is impropriety of factors suchas method and conditions of preparation of the compound, moldingconditions such as temperature, and post-molding conditions such ascooling condition.

Accordingly, an object of the present invention is to provide a rareearth bonded magnet which has a low porosity and which excels inmoldability, mechanical and magnetic characteristics and dimensionalstability, and to provide also a method, as well as a composition forrare earth bonded magnet, which enables easy manufacture of such a rareearth bonded magnet.

DISCLOSURE OF THE INVENTION

The present invention provides a method of manufacturing a rare earthbonded magnet formed by binding a rare earth magnetic powder by a binderresin, comprising the steps of:

mixing the magnet powder and the binder resin and kneading the mixtureso as to prepare a kneaded material;

granulating or graining the kneaded material to form the kneadedmaterial into a granular material;

conducting pressure molding on the granulated material at a firsttemperature at which the binder resin is softened or molten; and

cooling the molded body while keeping the molded body under pressure atleast in the period in which the molded body is cooled down to a secondtemperature which is below the first temperature.

This method enables production of a rare earth bonded magnet which has alow porosity and which excels in moldability, mechanicalcharacteristics, magnetic characteristics, and dimensional stability. Inparticular, it is to be noted that these advantageous features areobtainable with reduced amount of the binder resin.

Preferably, the binder resin is a thermoplastic resin.

The use of a thermoplastic resin further offers further improvement inthe moldability and further reduction of the porosity.

It is also preferred that the kneading is conducted at a temperature notlower than the thermal deformation temperature of the binder resin, suchthat the surfaces of the rare earth magnet powder particles are coatedwith molten or softened binder resin component.

This offers an improvement in the kneading efficiency so as to ensure amore uniform kneading, contributing to the reduction in the porosity.

Preferably, the content of the rare earth magnet powder in the kneadedmaterial ranges from 90 wt % to 99 wt %.

This contributes to improvement in the magnetic characteristics, becausethe binder resin content in the magnet is reduced correspondingly.

It is also preferred that the kneaded material contains an antioxidant.

The use of an antioxidant suppresses oxidation, degradation anddenaturation of the rare earth magnet powder and of the binder resin inthe course of the manufacture, thus contributing to the improvement inthe magnetic characteristics.

Preferably, the average granule size of the granular material rangesfrom 0.01 to 2 mm.

This ensures quantitative feed of the material while maintaining lowporosity, offering a high degree of dimensional precision of the rareearth bonded magnet.

Preferably, the second temperature is the melting temperature or thethermal deformation temperature of the binder resin.

It is also preferred that the pressure applied to the molded body duringcooling under pressure is maintained constant at least in the perioduntil the temperature falls to a temperature between the first andsecond temperatures.

These features render more remarkable the effects of reduction in theporosity and improvement in the dimensional precision.

The present invention also provides a method of manufacturing a rareearth bonded magnet formed by binding a rare earth magnetic powder by abinder resin, comprising the steps of: preparing a mixture or a kneadedblend of the rare earth magnet powder and the binder resin; granulatingor graining the mixture or kneaded blend to form a granular material;and conducting molding by using the granular material.

This method enables manufacture of a rare earth bonded magnet which hasa low porosity and which excels in moldability, magnetic characteristicand dimensional stability, i.e., dimensional precision. In particular,it is to be noted that these advantageous effects are obtainable evenwith reduced amount of the binder resin.

It is preferred that the maximum granule size of the granular materialis not greater than the minimum size of the gap in the mold used for themolding.

The maximum granule size of the granular material is preferably notsmaller than 0.02 mm.

The average granule size of the granular material preferably ranges from0.01 mm to 2 mm.

These features offer a further improvement in the dimensional precisionof the molded body, while maintaining the reduced porosity.

Preferably, the granulation or the graining is conducted by grinding.

The granulation or the graining can easily be performed by grinding.

Preferably, a heat treatment is conducted subsequent to the molding.

When the binder resin is a thermosetting resin, the heat treatmentserves to harden the uncured thermosetting resin, whereas, when thebinder resin is a thermoplastic resin, the heat treatment serves toenhance the binding force, whereby the mechanical strength of theproduct magnet is improved.

The present invention also provides a method of manufacturing a rareearth bonded magnet formed by binding a rare earth magnet powder by athermoplastic binder resin, comprising the steps of:

pressure-molding a composition containing the rare earth magnet powderand the binding resin at a first temperature at which the binder resinis softened or molten; and

cooling the molded body while keeping the molded body under pressure atleast in the period in which the molded body is cooled down to a secondtemperature which is below the first temperature.

The present invention also provides a method of manufacturing a rareearth bonded magnet formed by binding a rare earth magnet powder by athermoplastic binder resin, comprising the steps of:

kneading a composition containing the rare earth magnet powder and thebinding resin at a temperature which is not lower than the thermaldeformation temperature of the binder resin;

pressure-molding the kneaded material at a first temperature at whichthe binder resin is softened or molten; and

cooling the molded body while keeping the molded body under pressure atleast in the period in which the molded body is cooled down to a secondtemperature which is below the first temperature.

These methods of the invention enables production of rare earth bondedmagnets which have small porosity values and which excel in themoldability, mechanical characteristics, magnetic characteristics anddimensional stability. In particular, it is to be noted that theseadvantageous features are obtainable even with reduced amount of thebinder resin.

Preferably, the second temperature is the melting temperature or thethermal deformation temperature of the binder resin.

It is also preferred that the difference between the first and secondtemperatures is not smaller than 20° C.

These features render more remarkable the effects of reducing theporosity and improvement in the dimensional precision.

It is also preferred that the cooling under pressure is conductedcontinuously without releasing the pressure applied during the pressuremolding.

It is also preferred that the pressure applied during the cooling underpressure is equal to or lower than the pressure applied during thepressure molding.

It is also preferred that the pressure applied during the cooling underpressure is maintained constant at least in the period in which thetemperature comes down to the melting temperature of the binder resin.

The advantages brought about by the cooling under pressure are moreeffectively offered by these features, achieving further reduction inthe porosity and further improvement in the dimensional precision.

Preferably, the rate of cooling under pressure ranges from 0.5° C./secto 100° C./sec.

Such cooling rate serve to ensure high mechanical strength and highdimensional precision, without impairing production efficiency.

It is also preferred that the pressure applied during the pressuremolding is not higher than 60 kgf/mm².

Such a low pressure reduces burden on the mold and the molding machine,thus facilitating the production.

The present invention also provides a rare earth bonded magnetcomposition for use in the manufacture of a rare earth bonded magnetformed by binding a rare earth magnet powder by a binder resin, whereinthe composition is a granular material of a mixture or kneaded blend ofthe rare earth magnet powder and the binder resin, having an averagegranule size ranging from 0.01 mm and 2 mm.

The present invention further provides a rare earth bonded magnetcomposition for use in pressure molding for the manufacture of a rareearth bonded magnet formed by binding a rare earth magnet powder by abinder resin, wherein the composition is a granular material of amixture or kneaded blend of the rare earth magnet powder and the binderresin, having the maximum granule size not greater than the minimum sizeof the gap in the mold used in the pressure molding.

These rare earth bonded magnet composition permit easy manufacture ofrare earth bonded magnets which have low levels of porosity and whichexcel in moldability, mechanical characteristics, magneticcharacteristics and dimensional stability. In particular, it is to benoted that these advantageous effects are obtainable even with reducedamount of the binder resin.

Preferably, the maximum granule size of the granular material is notsmaller than 0.02 mm.

This effectively suppresses tendency for the porosity to increase.

The present invention further provides a rare earth bonded magnet formedthrough warm molding, wherein a thermoplastic binder resin softened ormolten during the warm molding has been cooled under pressure down to atemperature below the molding temperature so as to solidify to bind arare earth magnet powder, the magnet having a porosity not greater than4.5 vol %.

The present invention further provides a rare earth bonded magnet formedthrough warm molding, wherein a thermoplastic binder resin softened ormolten during the warm molding has been cooled under pressure down to atemperature not higher than the thermal deformation temperature or themelting temperature of the binder resin so as to solidify to bind a rareearth magnet powder, the magnet having a porosity not greater than 4.0vol %.

The present invention still further provides a rare earth bonded magnetformed through warm molding from a granular material prepared through agranulating step, wherein a thermoplastic binder resin softened ormolten during the warm molding has been cooled under pressure down to atemperature not higher than the thermal deformation temperature or themelting temperature of the binder resin so as to solidify to bind a rareearth magnet powder, the magnet having a porosity not greater than 4.0vol %.

These rare earth bonded magnets excel in the moldability, mechanicalcharacteristics, magnetic characteristics and dimensional stability. Inparticular, it is to be noted that these advantageous effects areobtainable even with reduced amounts of binder resins.

Preferably, the content of the rare earth magnet powder in the magnetranges from 92.0 wt % to 99.0 wt %.

Such rare earth magnet powder content correspondingly reduces the binderresin content in the magnet, achieving a further improvement in themagnetic characteristics.

The rare earth bonded magnet of the invention, when molded in theabsence of magnetic field, preferably exhibits maximum magnetic energyproduct (BH)max of not lower than 6 MGOe.

It is also preferred that the rare earth bonded magnet of the invention,when molded under the influence of a magnetic field, exhibits maximummagnetic energy product (BH)max of not lower than 12 MGOe.

Such superior magnetic characteristics offer excellent performance ofdevices such as motors incorporating the rare earth bonded magnets ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description will now be given of the method of manufacturinga rare earth bonded magnet, rare earth bonded magnet composition and arare earth bonded magnet.

The method of the present invention for manufacturing a rare earthbonded magnet has the following major steps.

<1> Preparation of Composition as Material of Rare Earth Bonded Magnet

The first step of the process is to prepare a composition which is to beused as the material of a rare earth bonded magnet (this compositionwill be referred to simply as "composition", hereinafter). Thecomposition is mainly composed of a rare earth magnet powder and abinder resin, and preferably contains an antioxidant. Other additivesare also contained as necessitated. These components are mixed togetherby means of a mixer such as a Henschel mixer or a blender, followed bykneading so as to form a kneaded compound.

A description will be given of each of these components.

1. Rare Earth Magnet Powder It is preferred that the rare earth magnetpowder comprises an alloy of a rare earth element and a transitionmetal. More specifically, the following Samples [1] to [5] arepreferred.

[1] A magnetic alloy powder containing, as basic elements, a rare earthelement R which is mainly constituted by Sm and a transition metal whichis mainly constituted by Co (referred to as "R--Co type alloy",hereinafter).

[2] A magnetic alloy powder containing, as basic elements, R (R is atleast one element which is selected from rare earth elements and whichincludes Y), a transition metal constituted mainly by Fe, and B(referred to as "R--Fe--B type alloy", hereinafter).

[3] A magnetic alloy powder containing, as basic elements, a rare earthelement R which is mainly constituted by Sm, a transition metal which ismainly constituted by Fe and an interstitial element which is mainly N(referred to as "R--Fe--N type alloy", hereinafter).

[4] A magnetic alloy powder containing, as basic elements, R (R is atleast one element which is selected from rare earth elements and whichincludes Y), and a transition metal constituted mainly by Fe, and havingmagnetic phases on the size of nanometers (referred to as"nano-crystalline magnet", hereinafter).

[5] A mixture of at least two of the compositions [1] to [4] set forthabove. Use of such a mixture permits easy achievement of excellentmagnetic characteristics by virtue of the combination of advantagesoffered by different types of magnet powder contained in the mixture.

Typical examples of the R--Co type alloy are SmCo₅ and Sm₂ TM₁₇, whereinTM represents a transition metal constituted mainly by Co.

Typical examples of the R--Fe--B type alloy are Nd--Fe--B alloys,Pr--Fe--B alloys, Nd--Pr--Fe--B alloys, Ce--Nd--Fe--B alloys,Ce--Pr--Nd--Fe--B alloys and other alloys which are obtained bysubstituting part of Fe of the foregoing alloys with other transitionmetal such as Co, Ni or the like.

A typical example of the R--Fe--N type alloy is Sm₂ Fe₁₇ N₃ which isprepared by nitriding Sm₂ Fe₁₇ alloy.

Examples of the rare earth elements in the magnet powder are Y, La, Ce,Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mish metals. One,two or more of the elements listed above may be contained. Examples ofthe transition metal are Fe, Co, Ni and the like. One, two or more ofthese metals may be contained. In order to improve the magneticcharacteristic, the magnet powder may contain, as required, an elementor elements such as B, Al, Mo, Cu, Ga, Si, Ti, Ta, Zr, Hf, Ag, Zn or thelike.

Although not exclusive, the average particle size of the magnet powderpreferably ranges from 0.5 to 100 μm and more preferably from 1 to 50μm. The average particle size of the magnet powder can be measured by,for example, F.S.S.S.(Fischer Sub-Sieve Sizer) method.

The particle sizes may be uniform around the average size or may bedistributed widely to some extent. Wide particle size distribution ispreferred when it is desired to obtain excellent moldability during themolding with the use of reduced amount of binder resin, as will bedescribed later. Such a wide distribution of the particle size providesa reduced porosity of a bonded magnet as the product. In case of themixture material [5] mentioned before, different kinds of magnet powderas the mixture components may have different average particle sizes.

No restriction is posed on the method of preparing the magnet powder.For instance, magnet powder is prepared by melt casting an alloy ingot,and pulverizing the alloy ingot into an appropriate size, followed bysieving. In an alternative method, a quenched ribbon (micro-finepolycrystalline structure) is formed by means of a quenching ribbonproduction apparatus of the type which is normally used for productionof amorphous alloys, and pulverizing the ribbon into particles ofsuitable sizes, followed by sieving.

The content of the magnet powder in the composition to be obtainedpreferably ranges from approximately 90 to 99 wt %, more preferably fromapproximately 92 to 99 wt % and most preferably from approximately 95 to99 wt %. A too small magnet powder content hampers improvement in themagnetic characteristics, in particular the maximum magnetic energyproduct, and tends to deteriorate the dimensional precision. Conversely,a too large magnet powder content causes a reduction in the moldability,due to a consequent increase in the binding resin content.

2. Binder resin

The binder resin may be either a thermoplastic resin or a thermosettingresin. The use of a thermoplastic resin as the binder resin provides anadvantage over the use of a thermosetting resin in that the magnet canhave a smaller porosity. According to the present invention, a furtherreduction in the porosity can be achieved, by virtue of the moldingtemperature condition and cooling condition which will be describedlater.

Examples of usable thermoplastic resins include polyamide resins (e.g.,nylon 6, nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12,nylon 6-12 and nylon 6-66); liquid crystal polymers such asthermoplastic polyimide resins and aromatic polyester resins;polyphenylene oxide resins; polyphenylene sulfide resins; polyolefinresins such as polyethylene resins, polypropylene resins andethylene-vinyl acetate copolymers; modified polyolefin resins; polyetherresins; polyacetal resins; and copolymers, mixtures, and polymer alloysof the foregoing resins. One of these resins is used alone or two ormore of these resins are used in the form of a mixture.

Among these resins, polyamide resins or their copolymers are preferablyused from the view points of moldability and mechanical strength,materials mainly constituted by liquid crystal polymers andpolyphenylene sulfides are used from the view point of heat resistance,and materials mainly constituted by polyolefins are preferably used fromthe view points of ease of molding and economy. These resins alsoexhibit superior kneadability with the magnet powder.

Preferably, the thermoplastic resin has a melting point of 120° C. orhigher, more preferably from 122° C. to 400° C. and most preferably from125° C. to 350° C. A melting point below the lower limit of the rangespecified above degrades the heat resistance of the molded magnet,making it difficult to obtain required levels of temperaturecharacteristics (magnetic or mechanical). Conversely, a melting pointexceeding the upper limit of the range specified above requires elevatedmolding temperature, tending to allow oxidation of the magnet powder.

In order to further improve the moldability, the thermoplastic used inthe invention preferably has an average molecular weight (polymerizationdegree) of from approximately 1000 to approximately 60000, morepreferably from approximately 12000 to 35000.

Examples of thermosetting resins suitably used are epoxy resins; phenolresins; urea resins; melamine resins; polyester resins (unsaturatedpolyester resins); polyether resins (e.g., polyether nitrile);polyurethane resins; and so forth. One of these resins may be used aloneor two or more of them may be used in the form of a mixture.

Among these resins, epoxy resins and phenol resins, in particular epoxyresins, are preferably used because they provide remarkable improvementin moldability, as well as superior mechanical strength. Thesethermoplastic resins excel also in the kneadability with magnet powder.

The thermosetting resins, when used, are added in a state in which theyhave not yet been hardened, and may be in solid (powdered), liquid orsemi-liquid phase at room temperature.

The content of the binding resin in the composition preferably rangesfrom approximately 1 wt % to approximately 10 wt %, more preferably fromapproximately 1 wt % to approximately 8 wt %, and most preferably fromapproximately 1 wt % to approximately 5 wt %. A too large content of thebinder resin does not contribute to improvement in the magneticcharacteristics (in particular, maximum magnetic energy product) but,rather, tends to deteriorate the dimensional precision. Conversely, atoo small binder resin content impairs moldability.

3. Antioxidant

Antioxidant is an additive which is employed in the preparation of thecomposition of the present invention for the purpose of preventingoxidation deterioration of the rare earth magnet powder and denaturationof the same due to oxidation of the binder resin (this denaturationoccurs as a result of catalytic action of the metallic components of therare earth magnetic powder). The addition of the antioxidant iseffective in preventing oxidation of the rare earth magnet powder andcontributes to improvement in the magnetic characteristic of the magnet,as well as to improvement in the thermal stability of the compositionunder kneading and molding. Thus, the antioxidant plays an importantrole of ensuring good moldability with reduced amount of the binderresin.

The antioxidant tends to be lost by evaporation or denaturated duringkneading and molding into a magnet, only part of the antioxidant addedremains and exists in the rare earth bonded magnet as the product.

Any type of anti-oxidant which can prevent or suppress oxidation of therare earth magnet powder may be used. For instance, suitably used arechelating agents which form chelate compounds with the metallic ions, inparticular Fe ions, such as amine type compounds, amino-acid typecompounds, nitrocarboxylic acid type compounds, hydrazine typecompounds, cyanide type compounds, sulfide type compounds and so forth.Obviously, antioxidants mentioned above are only illustrative andantioxidants of other types and compositions than those listed above maybe used.

The content of the antioxidant when it is used in the compositionpreferably ranges from approximately 0.1 wt % to approximately 2 wt %,more preferably from approximately 0.5 wt % to 1.5 wt %. At the sametime, the ratio of the antioxidant content to the binder resin contentpreferably ranges from approximately 2 percent to approximately 150percent, more preferably from approximately 30 percent to approximately100%.

In the present invention, the degree of oxidation of the magnet powder,as well as the degree of denaturation of the same due to oxidation ofthe binder resin, is ruled by the ratio of the content between themagnet powder and the binder resin. Needless to say, therefore, thecomposition of the invention may have an antioxidant content which fallsbelow the range set forth above or may even be completely devoid of suchantioxidant.

The contents of the binder resin and the antioxidant are determinedtaking into account, for example, the following factors.

A comparatively small binder resin content causes the magnet powdercontent to increase correspondingly, so that the viscosity of themixture under kneading is enhanced to require a greater kneading torque,resulting in enhanced generation of heat and resultant promotion ofoxidation. If the content of the antioxidant is small, oxidation of theresin cannot be sufficiently suppressed, which serves to increase theviscosity of the material under kneading (molten resin), with theresults that the kneadability and moldability are impaired to hamperproduction of magnets which have low porosity and which excel inmechanical strength and dimensional stability. In contrast, when thecontent of the antioxidant is comparatively large, the binder resincontent decreases correspondingly, tending to reduce the mechanicalstrength of the molded green body.

Conversely, a greater binder resin content correspondingly decreases thecontent of the magnet powder, which in turn reduces the influence of themagnetic powder on the resin, thus suppressing oxidation tendency of theresin. it is therefore possible to suppress oxidation of the resin evenwith reduced amount of the antioxidant.

Thus, a comparatively large content of the binder resin permit areduction in the content of the antioxidant, whereas a smaller binderresin content requires the use of a greater amount of antioxidant.

Thus, the total content of the binder resin and the antioxidant in thecomposition preferably ranges from 1.0 wt % to 8.0 wt %, more preferablyfrom 2.0 wt % to 6.0 wt %. A total content falling within the rangespecified above contributes to improvement in the moldability and toenhancement of the anti-oxidation effect, thus offering advantages inthe production of magnets having low porosity and excellent both inmechanical strength and magnetic characteristics.

4. Other additives

The composition may contain, as necessitated, other type or types ofadditives such as a plasticizer (e.g., aliphatic acid salt such as zincstearate, and aliphatic acid such as oleic acid), a lubricant (e.g.,silicone oil, wax, aliphatic acid, and inorganic lubricant such asalumina, silica and titania), a hardening agent, a hardening assistant,and other molding assistants.

Addition of the plasticizer improves fluidity during the molding, sothat desired characteristics can be achieved with the use of a reducedamount of binder resin. In addition, pressure molding can be carried outwith a lower compacting pressure. These advantages are also obtainablethrough the use of a lubricant. Preferably, the content of theplasticizer is from approximately 0.01 wt % to approximately 0.2 wt %,while the content of lubricant is preferably from approximately 0.05 wt% to approximately 0.5 wt %.

The rare earth magnet powder and the binder resin, preferably togetherwith an antioxidant and together with other additives as required, aremixed to form a mixture which is used as the magnet molding composition.The mixing is conducted by using, for example, a mixer such as aHenschel mixer, or a blender.

Preferably, the mixture is further kneaded to form a kneaded materialwhich is used as the magnet molding composition. Such kneading may beperformed by, for example, a twin-screw extrusion kneader, a roll-typekneader, or an ordinary kneader.

The kneading may be conducted at a room temperature but is preferablyconducted at a temperature not lower than the thermal deformationtemperature of the binder resin (measured by method specified by ASTMD648) or not lower than the softening point of the same, more preferablynot lower than the melting point of the binder resin.

For instance, when a polyamide resin (thermal deformation temperature145° C., melting point 178° C.) is used as the binder resin, kneadingtemperature specifically preferred is from approximately 150° C. toapproximately 280° C., whereas, when a phenol novolak resin (softeningpoint 80° C.) is used, specifically preferred kneading temperature isfrom approximately 80° C. to approximately 150° C.

The kneading time varies according to conditions such as the kind of thebinder resin, type of the kneader used and the kneading temperature.Usually, however, the kneading time preferably is from about 3 minutesto about 120 minutes, more preferably from about 5 minutes to about 40minutes.

The kneading should be conducted sufficiently to obtain such a statethat the surface of the powder particle of the rare earth magnet powderis coated with the molten or softened binder resin component. In orderto obtain the above-mentioned state, when kneading is conducted at atemperature falling within the range specified above, the kneading timepreferably ranges from approximately 5 minutes to 90 minutes, morepreferably from approximately 5 minutes to 60 minutes, although thekneading time may vary according to factors such as the kind of thebinder resin, type of the kneading machine used and the kneadingtemperature.

The described kneading conditions serve to improve kneading efficiencyand permits uniform kneading in a shorter time than that required whenthe kneading is conducted at room temperature. At the same time, since acomparatively low viscosity of the binder resin is maintained during thekneading, the particles of the rare earth magnet powder are uniformlycoated by the binder resin. This contributes to a reduction in theporosity of the composition and, hence, the porosity in the magnet asthe product.

When n types of thermoplastic resins (n being an integer) are used inthe form of a mixture, the aforesaid "thermal deformation temperature(or melting point) of the binder resin used" can be determined throughthe following calculation.

It is assumed that the total amount of the thermoplastic resins is oneweight part. Amounts of the respective thermoplastic resins areexpressed by A₁, A₂ . . . , A_(n), respectively, while the thermaldeformation temperatures (or melting points) of the respectivethermoplastic resins are expressed by T₁, T₂, . . . , T_(n). The thermaldeformation temperature (or melting point) of the thermoplastic resinmixture is given by A₁ T₁ +A₂ T₂ + . . . A_(n) Tn. This calculationmethod applies also to the cases where n types of thermoplastic resinsare used in the form of a mixture.

<2> Preparation of granules

The mixture or kneaded composition prepared through compositionpreparation step <1> described above is then granulated or pelletized soas to become granules of a predetermined grain size.

Although no restriction is posed on the method of granulation orpelletizing, the granulation or pelletizing is preferably conducted bycrushing the kneaded composition. The crushing may be effected by, forexample, a ball mill, a vibration mill, a breaker, a jet mill or a pinmill.

The granules also may be prepared by using an extrusion pelletizer oreven by a combination of the pelletizing and crushing.

The grain size of the granules may be regulated through classificationby means of, for example, a sieve.

A description will now be given of the suitable size of the granules.

Preferably, the maximum size of the granules is not greater than theminimum size of the mold gap (space to be filled with the granules).Specifically, the maximum size of the granules is not smaller than 0.02mm, preferably not smaller than 0.05 mm. Maximum granule size exceedingthe mold gap hinders the filling of the mold with the granules or makesit difficult to control the amount of the granules to be charged, thushampering improvement in the dimensional precision of the bonded magnet.Conversely, a too small maximum size of the granules tends to increasethe porosity of the bonded magnet as the product.

Preferably, the granules have an average size of from approximately 0.01mm to approximately 2 mm, more preferably from approximately 0.02 mm toapproximately 2 mm, and most preferably from approximately 0.05 mm toapproximately 2 mm. Average granule size exceeding 2 mm makes itdifficult to delicately control the amount of the granules charged inthe mold, thus impairing quantitative control, failing to achieve aimedimprovement in the dimensional precision, particularly when thedimensions of the magnet to be manufactured are small, i.e., when thesize of the mold gap is small. On the other hand, granules of an averagesize less than 0.01 mm are not easy to prepare or requires a troublesomework in the preparation. In addition, a too small average granule sizetends to increase the porosity of the bonded magnet as the product.

The granule size is preferably uniform, although it may have adistribution to some extent. Such a uniform granule size enhances thepacking density in the mold, leading to low porosity and highdimensional precision of the product bonded magnet.

<3> Pressure Molding

Pressure molding is performed on the rare earth bonded magnetcomposition, in particular the granules prepared in the preceding step<2> of preparing granules. A description will now be given of a pressuremolding which is a representative example of the pressure molding.

A predetermined amount of granules of the rare earth bonded magnetcomposition, obtained through a volumetric measurement such as, forexample, grading method (a method in which a powder-like material ischarged in a vessel by gravity and any surplus material protruding abovethe top of the vessel is scraped off) or through weighing, is charged ina mold gap of a pressure molding machine.

The granular material thus charged in the mold is then molded bypressure with or without application of an aligning magnetic field. (Thealigning magnetic field intensity may be, for example, from 5 to 20 KOe,and the aligning direction may be longitudinal, transverse or radial.)The pressure molding may be cold molding (molding at or around roomtemperature), low-temperature warm molding (molding conducted by heatingthe composition to a temperature below the softening point of the binderresin), or warm molding, although warm molding is preferred. Namely,suitable treatment such as heating of the mold is conducted so as toelevate the temperature of the material under molding to a predeterminedtemperature (referred to as "first temperature", hereinafter) at whichthe binder resin, specifically a thermoplastic resin, is softened ormolten.

Thus, the first temperature is a temperature which is not lower than thethermal deformation temperature or not lower than the softeningtemperature of the binder resin used. More specifically, when the binderresin used is a thermoplastic resin, the first temperature is preferablynot lower than the melting temperature of the resin, more preferably itranges from the melting temperature to the temperature which is 200° C.above the melting temperature, and most preferably from the meltingtemperature to the temperature which is 130° C. above the meltingtemperature.

For instance, when the thermoplastic resin used is a polyamide resin(melting temperature 178° C.), the preferred material temperature underthe molding, i.e., the first temperature, should range from about 180°C. to about 300° C. In contrast, when the binder resin used is phenolnovolak resin (softening temperature 80° C.) which is a thermosettingresin, the first temperature preferably falls within the range of from80° C. to 280° C.

Pressure molding, when conducted at such a temperature, ensured highfluidity of the material under molding, thus enabling mass-production ofmagnets having low porosity, high mechanical strength and high degreesof shape and dimensional stability, even when the magnet is of a shapehaving thin-walled portion, e.g., ring-shape, tabular shape or curvedtabular shape, and even when the magnet is small in size or elongated,not to mention ordinary cylindrical or block shape.

The compacting pressure employed in the pressure molding is preferably60 kgf/mm² or less, more preferably from approximately 2 kgf/mm² toapproximately 50 kgf/mm², and most preferably from approximately 5kgf/mm² to approximately 40 kgf/mm². According to the present invention,it is possible to shape and mold bonded magnets having foregoingadvantageous features, even at such comparatively low levels of moldingpressure, by virtue of the fact that the molding is conducted at thefirst temperature set forth above.

Such low levels of molding pressure correspondingly reduces loads on themold and the molding machine, eliminating necessity of use of mold andmolding machines having greater size and power, while extending thelives of the same, thus offering an advantage in the manufacturingprocess.

When the pressure molding is carried out by cold molding technique, themolding pressure preferably ranges from approximately 20 kgf/mm² toapproximately 100 kgf/mm², and more preferably from approximately 30kgf/mm² to approximately 70 kgf/mm².

<4> Cooling

When the pressure molding is warm molding, the molded body should becooled. Preferably, the cooling is conducted down to a predeterminedtemperature (referred to as "second temperature") which is below thefirst temperature, while keeping the molded body under a pressure. Thiscooling step, therefore, will be referred to as "cooling underpressure", hereinafter.

Such cooling under pressure maintains the low porosity established inthe course of the molding without change, thus ensuring low porosity,high dimensional stability and superior magnetic characteristics of therare earth bonded magnet as the product.

In order to achieve low porosity and high dimensional stability of theproduct bonded magnet, it is preferred that the second temperature(pressure releasing temperature) is set to be low as possible. Accordingto the present invention, the second pressure is preferably not higherthan the meting temperature of the binder resin (specificallythermoplastic resin), and more preferably not higher than the thermaldeformation temperature (softening temperature) of the resin used as thebinder resin.

Preferably, the difference between the first temperature and the secondtemperature is 20° C. or greater, more preferably 50° C. or greater. Thegreater the difference, the greater the effects in reducing porosity andenhancing dimensional precision.

A comparatively large magnet powder content in the composition permitseasier production of low-porosity bonded magnet, even if the secondtemperature is set to a comparatively high level. For instance, when themagnet powder content in the kneaded material is 94% or greater, it ispossible to obtain a low porosity (not greater than 4.5% or not greaterthan 4.0%), even when the second temperature is set to a temperaturearound the melting point or a temperature which is about 10° C. higherthan the melting temperature of the binder resin.

The cooling under pressure may be executed after a temporal dismissal orreduction of the pressure from the molding pressure. For the purpose ofsimplification of the process and improvement in the dimensionalprecision, however, it is preferred that the cooling is conductedwithout dismissing the molding pressure.

The pressure may be changed during the cooling. It is, however,preferred that the pressure is maintained constant at least in theperiod in which the temperature falls from the first temperature to thesecond temperature. More preferably, the pressure is maintained constantat least in the period in which the material temperature falls down tothe melting temperature, still more particularly down to the thermaldeformation temperature, of the resin used as the binder resin. Suchcontrol of the pressure offers a remarkable effect in the reduction ofthe porosity and enhancement of the dimensional precision.

The change of the pressure during the cooling may be controlled inaccordance with such a pattern that the pressure increases or decreaseslinearly or stepwise.

The pressure (average pressure when the pressure changes by time)applied during the cooling under pressure should be equal to or belowthe molding pressure applied during the pressure molding. Morepreferably, the same level of pressure as the molding pressure ismaintained at the shortest until the temperature comes down to themelting temperature of the binder resin. When any pressure is applied inthe period in which the material cools from the melting temperature tothe thermal deformation temperature of the binder resin, the pressure insuch a period is preferably set to range from approximately 40% toapproximately 100%, more preferably from approximately 50% to 80% of themolding pressure. The effects of reducing porosity and enhancingdimensional precision are rendered more remarkable by the describedcontrol of pressure.

Needless to say, the present invention does not exclude continuation ofthe cooling without application of pressure, i.e., under normalatmospheric pressure, subsequent to the cooling under pressure, i.e.,after the release of the pressure. It is also possible to conduct,subsequent to the cooling without application of pressure, alater-mentioned heat treatment, followed by another cooling underpressure.

There is no restriction in the cooling rate (average cooling rate whenthe rate changes by time) during the cooling under pressure. The coolingrate, however, ranges preferably from 0.5° C./sec to 100° C./sec, morepreferably from 1° C./sec to 80° C./sec. A too high cooling rateinvolves a risk of fine cracking in the molded body due to rapidcontraction caused by the quick cooling, resulting in a reduction of themechanical strength. Too quick cooling also enhances the internal stresswhich, when the molded body is removed from the mold, causes distortionor deformation of the body due to relaxation of the stress, leading toan inferior dimensional precision. Conversely, a too low cooling rateprolongs the cycle time, resulting in a reduction of the productivity.

The rate of cooling after release of the pressure, if such cooling isconducted, may be done at any desired rate. Thus, the foregoing rangesof the cooling rate may apply to such cooling conducted after release ofthe pressure.

The rate of cooling under pressure, as well as the rate of coolingexecuted after release of pressure, may be constant or varied inrelation to time.

The cooling may be effected by any suitable method such as naturalcooling, forced air cooling, water cooling, oil cooling, or acombination of water cooling and air cooling.

<Heat Treatment>

The cooling step <4> described above need not be executed when themolding in the foregoing step <3> is conducted by cold molding. In sucha case, the molded body is subjected as required to a heat treatment(baking). Such a heat treatment may also be conducted after warm moldingor low-temperature warm molding or after the cooling subsequent to suchmolding.

The primary purpose of the heat treatment is to thermally cure thebinder resin when the latter is a thermosetting resin, while thesecondary purpose is to soften or melt the binder resin so as to enhancethe bonding force thereby increasing the mechanical strength.

The heat treatment intended for the primary purpose is to heat thethermosetting binder resin to a temperature not lower than the hardeningtemperature, and requires a comparatively long heating time of, forexample, from 30 minutes to 4 hours.

The heat treatment intended for the secondary purpose may be executed toproduce appreciable effect not only when the pressure molding is carriedout as cold molding but also when the same is conducted by warm moldingor low-temperature warm molding. This heat treatment is conducted byheating the binder resin to a temperature not lower than the softeningtemperature, preferably not lower than the melting temperature, and theheating time may be as short as from, for example, one to 30 minutes orso.

This heat-treating step may be executed in an overlap with the foregoingstep <3> or subsequent to the step <3>, or even after the cooling step<4>.

Obviously, the invention does not exclude any heat treatment which isintended for purposes other than the foregoing primary and secondarypurposes.

The rare earth bonded magnets of the present invention, manufactured bythe method as described, exhibit the following excellentcharacteristics. More specifically, according to the invention, the rareearth bonded magnet of the present invention can have a low porosity,preferably 4.5 vol % or less, more preferably 4.0 vol % or less and mostpreferably 3.0 vol % or less. Such low porosity, i.e., high density,offers greater mechanical strength, as well as improved corrosionresistance, and provides a high dimensional precision which ensuredreduced fluctuation of dimensions, thus realizing superior dimensionalstability.

The rare earth bonded magnet in accordance with the present inventionalso excels in the magnetic characteristics. Even isotropic magnetsexhibit superior magnetic characteristics by virtue of the compositionof the magnet powder and the large content of the magnet powder.

The content of the rare earth magnet powder in the rare earth bondedmagnet of the present invention ranges preferably from approximately 92wt % to approximately 99 wt %, more preferably from approximately 94 wt% to approximately 99 wt %, and most preferably from approximately 94 wt% to approximately 99 wt %. A too small content of the magnet powderdoes not contribute to improvement in the magnetic characteristic (inparticular maximum magnetic energy product) and tends to impair thedimensional precision. Conversely, a too large content of the magnetpowder causes the binder resin content to decrease correspondingly,resulting in an inferior moldability.

The rare earth bonded magnet of the invention, when manufactured withoutmagnetic field, preferably possesses maximum magnetic energy product(BH)max of 6 MGOe or greater, more preferably 8 MGOe or greater. Whenmanufactured under the influence of a magnetic field, the magnetpossesses maximum magnetic energy product (BH)max of 12 MGOe or greater,more preferably 13 MGOe or greater. The rare earth bonded magnet of thepresent invention, which has the superior magnetic characteristics andhigh dimensional precision as described, offers excellent performance ofa device such as a motor incorporating this magnet.

No restriction is posed in regard to the shape and dimensions of therare earth bonded magnet of the present invention. Thus, the magnet canhave any desired shape or configuration such as cylindrical shape,polygonal columnar shape, tubulAr shape, arcuate shape, flat tabularshape or curved tabular shape. The size may vary from large to small.Furthermore, there is no limitation in the purpose of use of the rareearth bonded magnet of the present invention.

Examples of the present invention will now be described.

EXAMPLE 1

The following magnet powder, binder resin (thermoplastic resin) andadditive were mixed together to form a mixture which was subjected tokneading, and the kneaded matter was granulated (grained) into granules.The granular material was charged in a mold of a molding machine and waspressure-molded (warm molding) without application of magnetic field.After the molding, cooling was executed while maintaining the same levelof pressure as the molding pressure, whereby Sample Nos. 1a to 9a ofrare earth bonded magnets, having magnet powder particles bondedtogether by the solidified binder resin, were obtained. The amounts ofthe matters shown below are expressed in terms of the contents in thekneaded composition.

Composition

Nd--Fe--B type magnet powder:

Nd₁₂.0 Fe₇₇.8 Co₄.3 B₅.9

96.0 wt % (content in product magnet is almost the same as this value)

Thermoplastic resin:

Each of resins A to G shown in Table 1, 2.8 wt %

Antioxidant: hydrazine-type antioxidant, 1.2 wt %

Mixing: Mixed by means of Henschel mixer

Kneading: Kneaded by a twin-screw kneader at temperatures shown in Table2. Screw speed 100 to 300 r.p.m., kneading time (time of stay in thekneader) 5 to 15 minutes

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 0.8mm, though grinding and classification.

Molding: Granulated material was charged into a mold at roomtemperature, and pressure-molded when temperature has been raised to apredetermined molding temperature (first temperature). As to moldingtemperature and molding pressure, reference be made to Table 2.

Cooling: The molded body was cooled down to a predetermined pressurereleasing temperature (second temperature) while the pressure wasmaintained. After the release of the pressure, cooling was continueddown to the room temperature. Samples were thus obtained and removedfrom the molds. As to the pressure releasing temperature, reference bemade to Table 2. The cooling rate in the cooling under pressure was 1°C./sec.

Product shapes:

Cylindrical shape (outside diameter 30 mm×inside diameter 28 mm×height 7mm)

Flat tabular shape (20 mm wide×20 mm long×3 mm thick) (For use as amechanical strength test piece)

The values of the thermal deformation temperature appearing in Table 1were those obtained through measurement conducted in accordance withASTM D 648 set forth below.

ASTM D648: A test piece in an oil bath is supported at its both ends andloaded at its center by a loading bar so as to have a bending stress of4.6 kgf/cm².

Oil temperature is raised at a rate of 2° C./minute, and the temperatureat which the deflection reaches 0.254 mm is measured.

Magnetic performance (magnetic flux density Br, coercive force iHc,maximum magnetic energy product (BH)max), as well as density, porosity,mechanical strength and corrosion resistance were examined with thesamples of the rare earth bonded magnet thus obtained. The results areshown in Table 3.

The items of evaluation appearing in Table 3 were measured in accordancewith the following methods.

Magnetic performance:

Each sample was magnetized by 40 KOe pulse and subjected to measurementby a D.C. magnetometer under application of maximum magnetic field of 25KOe. Alternatively, magnet piece of 5 mm square and 1 mm thick was cutout of each sample and was subjected to measurement by a vibrationspecimen magnetometer (VSM).

Density:

Density was measured in accordance with Archimedean method (submergingmethod).

Porosity:

Porosity was calculated from the weighed composition and the measureddensity of the molded body.

Mechanical strength:

Mechanical strength was measured through punching shearing test. Amachine named autograph, manufactured by Shimadzu Corporation, was usedas the testing machine. The test was conducted at a shearing rate of 1.0mm/min, by using a circular punch 3 mm diameter. Flat tabular magnet wasused as the test piece.

Corrosion resistance:

Molded magnet was placed in a thermo-humidistat oven maintaining anatmosphere of 80° C. and 90% humidity, and the time until rust isgenerated was measured. The sample magnets were taken out of the oven atevery 50 hours for the observation of their surfaces with an opticalmicroscope (magnification 10). After elapse of 500 hours, theobservation interval was changed to 500 hours.

As will be clear from Table 3, Sample Nos. 1a to 9a of the rare earthbonded magnet of the present invention, manufactured by using athermoplastic resin as the binder resin, have porosity values as smallas 1% or less, despite the low molding pressure employed in the molding.Thus, the bonded magnet in accordance with the invention has a highdensity which approximates stoichiometric density and, accordingly, anextremely high mechanical strength.

Furthermore, these samples of the bonded magnet showed sufficiently highcorrosion resistance, even with no coating thereon. This is consideredto be attributed to small porosity which ensures that the binder resinuniformly covers the magnet powder particles.

Sample Nos. 1a to 9a of the bonded magnet were cut and the cut surfaceswere observed through electron-microscopic photography (S EM). The cutsurface of each sample showed almost no pore, and it was confirmed alsothat the binder resin was uniformly distributed around the particles ofthe magnet powder particles.

It will also be seen that the samples of the bonded magnet of theinvention exhibits high levels of magnetic flux density Br, coerciveforce iHc and maximum magnetic energy product (BH)max, thusdemonstrating excellent magnetic characteristics.

EXAMPLE 2

The following magnet powder and binder resin (thermosetting resin) weremixed together to form a mixture which was subjected to kneading, andthe kneaded matter was granulated (grained) into granules. The granularmaterial was charged in a mold of a molding machine at the roomtemperature and was pressure-molded (cold or warm molding) withoutapplication of magnetic field. After the molding, binding resin washardened, whereby Sample Nos. 10a to 15a of rare earth bonded magnetswere obtained. The amounts of the matters shown below are expressed interms of the contents in the kneaded composition.

Composition

Nd--Fe--B type magnet powder:

Nd₁₂.0 Fe₇₇.8 CO₄.3 B₅.9

96.0 wt % (content in product magnet is almost the same as this value)

Thermosetting resin:

Each of resins shown in Table 4

4.0 wt % (inclusive of hardening agent)

Mixing:

Mixed by means of a twin-cylinder mixer when the binder resin used wassolid at room temperature. A blender was used when the binder resin wasin liquid phase at the room temperature.

Kneading:

Kneaded by a kneader at temperatures shown in Table 5. Screw speed 50 to250 r.p.m., kneading time 30 minutes

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 0.8mm or smaller, through grinding and classification.

Molding:

Granulated material was charged into a mold at room temperature, andpressure-molded at a predetermined molding temperature. As to moldingtemperature and molding pressure, reference be made to Table 5.

Cooling:

The molded body was cooled down to a predetermined pressure releasingtemperature and, after release of the pressure further cooled c down tothe room temperature except for Sample Nos. 10a and 11a. Samples werethus obtained and removed from the molds. The cooling was conducted byair cooling. As to the pressure releasing temperature, reference be madeto Table 5. The cooling rate was 2° C./sec.

Heat treatment:

The tentative molded samples were placed in a thermostat oven, for thepurpose of hardening the thermosetting resin. As to the hardeningconditions, refer to Table 4.

Product shapes:

Cylindrical shape (outside diameter 30 mm×inside diameter 28 mm×height 7mm)

Flat tabular shape (20 mm wide×20 mm long×3 mm thick) (For use as amechanical strength test piece)

Surface treatment:

A 10 μm thick coating of epoxy resin was formed by spraying on eachsample of the molded bodies to be subjected to a corrosion resistancetest.

Magnetic performance (maximum magnetic energy product (BH)max), as wellas density, porosity, mechanical strength and corrosion resistance wereexamined with the samples of the rare earth bonded magnet thus obtained.The results are shown in Table 6. The method of evaluation of each itemis the same as that in Example 1.

Sample Nos. 10a to 15a of the bonded magnet were cut and the cutsurfaces were observed through electron-microscopic photography. It wasconfirmed also that the binder resin was uniformly distributed aroundthe particles of the magnet powder particles, although not few poreswere observed in the core of the magnet.

It will be seen from Tables 5 and 6 that the rare earth bonded magnet(in particular Sample Nos. 10a to 15a), manufactured by usingthermosetting binder resins, requires higher levels of compactingpressure than those employed in the manufacture of the magnet usingthermoplastic binder resins and exhibits correspondingly increasedporosity. The increased porosity, however, is still as small as from 5to 6% or so.

As to the corrosion resistance test, although the samples required asurface treatment, such a surface treatment may be light one, i.e., onlya thin coating film had to be formed, and the samples showed extremelyexcellent corrosion resistance even with such light surface treatment.This is attributable to the small porosity, as well as to the uniformdistribution of the binder resin around the particles of the magnetpowder as confirmed through the SEM observation. Namely, the magnetpowder particles are uniformly coated with the binder resin which alsoserves to increase adhesion of the magnet powder particles. Since thedegree of the required surface treatment is not substantial, highdimensional precision is maintained on the whole magnet as the product.

EXAMPLE 3

The following magnet powder, binder resin (thermoplastic resin) andadditive were mixed together to form a mixture which was subjected tokneading, and the kneaded matter was granulated (grained) into granules.The granular material was charged in a mold of a molding machine and waspressure-molded (warm molding) under influence of a magnetic field.After the molding, cooling was executed while maintaining the same levelof pressure as the molding pressure, whereby Sample Nos. 16a to 19a ofrare earth bonded magnets were obtained. The amounts of the mattersshown below are expressed in terms of the contents in the kneadedcomposition.

Composition

Sm--Co type magnet powder:

Sm(Co_(bal). Fe₀.32 Cuo₀.6 Zr₀.016)₇.8

95.0 wt % (content in product magnet is almost the same as this value)

Thermoplastic resin: PPS resin, 4.2 wt %

Antioxidant: hydrazine-type antioxidant, 0.8 wt %

Mixing: Mixed by means of twin-cylinder mixer

Kneading: Kneaded by using various types of kneading machines. As tokneading conditions, refer to Table 7.

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 0.8mm, through grinding and classification.

Molding:

Granulated material was charged into a mold at room temperature, andpressure-molded under the influence of a transverse magnetic field (15KOe) when temperature has been raised to a predetermined moldingtemperature (first temperature). The molding temperature was 320° C.,while the molding pressure was 20 kgf/mm².

Cooling:

The molded body was cooled down to a predetermined pressure releasingtemperature (second temperature) which was 150° C. while the pressurewas maintained. After the release of the pressure, cooling was continueddown to the room temperature. Samples were removed from the molds afterdemagnetization. The cooling was conducted by air cooling. The coolingrate in the cooling under pressure was 5° C./sec.

Product shapes:

Rectangular parallelopiped (11 mm long×8 mm wide×7 mm high, aligned inheightwise direction)

Flat tabular shape (20 mm wide×20 mm long×3 mm thick) (For use as amechanical strength test piece)

Magnetic performance (maximum magnetic energy product (BH)max), as wellas density, porosity, mechanical strength and corrosion resistance wereexamined with the samples of the rare earth magnet powder thus obtained.The results are shown in Table 8. Methods of evaluations of the itemsare the same as those in Example 1.

As will be clear from Table 8, Sample Nos. 16a to 19a of the rare earthbonded magnet of the present invention have porosity values as small as1% or less. Thus, the bonded magnet in accordance with the invention hasa high density and, accordingly, high levels of mechanical strength andcorrosion resistance.

Furthermore, Sample Nos. 16a to 19a were subjected to SEM observationsimilar to those of preceding Examples. Almost no pore was observed, andit was confirmed also that the binder resin was uniformly distributedaround the particles of the magnet powder particles.

it is also understood that the maximum magnetic energy product (BH)maxis large, thus demonstrating superior magnetic characteristics.

COMPARATIVE EXAMPLE 1

The following magnet powder, binder resin (thermoplastic resin) andadditive were mixed together to form a mixture which was then charged ina mold of a molding machine and was pressure-molded (warm molding) underinfluence of a magnetic field. After the molding, cooling was executedwithout applying pressure, whereby Sample Nos. 20a and 21a of rare earthbonded magnets were obtained. The amounts of the matters shown below areexpressed in terms of the contents in the mixture.

Composition

Sm--Co type magnet powder:

Sm(Co_(bal). Fe₀.32 Cu₀.06 Zr₀.016)₇.8

95.0 wt % (Sample No. 20a)

96.0 wt % (Sample No. 21a)

Thermoplastic resin:

PPS resin 4.2 wt % (Sample No. 20a)

3.2 wt % (Sample No. 21a)

Antioxidant: hydrazine-type antioxidant 0.8 wt %

Mixing: Mixed by means of twin-cylinder mixer

Molding:

The mixture was charged into a mold at room temperature, andpressure-molded under the influence of a transverse magnetic field (15KOe) when temperature has been raised to a predetermined moldingtemperature. The molding temperature was 320° C., while the moldingpressure was 20 kgf/mm².

Cooling:

The molded body was demagnetized in the mold and was taken out of themold, followed by cooled down to normal temperature under atmosphericpressure. The cooling was conducted by air cooling. The cooling rate inthe cooling under pressure was 5° C./sec.

Product shapes:

Rectangular parallelopiped (11 mm long×8 mm wide×7 mm high, aligned inheightwise direction)

Flat tabular shape (20 mm wide×20 mm long×3 mm thick) (For use as amechanical strength test piece)

Sample Nos. 20a and 21 a of the magnet could not be formed into thedesired shapes due to recession of the molded body and breakage of edgesof the same, caused by deposition of the edge and end surfaces of themolded body to the punch of the molding machine, as a result of leakageof the resin during the molding.

SEM observation of the molded bodies showed non-uniform distribution ofthe binder resin, allowing mixing of discrete magnet powder and bindingresin. Many pores were also observed.

Both Sample Nos. 20a and 21a were defective as stated above, and couldnot effectively be subjected to measurement of characteristics such asmechanical strength.

COMPARATIVE EXAMPLE 2

The following magnet powder, binder resin (thermosetting resin) andadditive were mixed together to form a mixture which was then charged ina mold of a molding machine and was pressure-molded (cold molding) underinfluence of a magnetic field. After the molding, the resin washardened, whereby Sample No. 22a of rare earth bonded magnets wasobtained. The amounts of the matters shown below are expressed in termsof the contents in the mixture.

Composition

Sm--Co type magnet powder:

Sm(Co_(bal). Fe₀.32 Cu₀.06 Zr₀.016)₇.8

96.0 wt %

Thermosetting resin:

Bisphenol A novolak resin (melting temperature 60° C.)

3.6 wt % (inclusive of hardening agent)

Antioxidant: Hydrazine-type antioxidant 0.4 wt %

Mixing: Mixed by means of twin-cylinder mixer

Molding:

The mixture was charged into a mold at room temperature, andpressure-molded under the influence of a transverse magnetic field (15KOe).

The molding was conducted at the room temperature, while the moldingpressure was 20 kgf/mm².

Heat treatment:

The molded body was demagnetized in the mold and then taken out of themold. The molded body was then subjected to a heat treatment conductedat 170° C. for 4 hours, so as to harden the thermosetting resin.

Product shapes:

Rectangular parallelopiped (11 mm long×8 mm wide×7 mm high, aligned inheightwise direction)

Flat tabular shape (20 mm wide×20 mm long×3 mm thick) (For use as amechanical strength test piece)

Sample No. 22a of the magnet could not be formed into the desired shapesdue to dropping of the magnet powder, recession of the molded body andbreakage of edges of the same. This is attributed to small bonding forcebetween magnet powder particles, insufficient distribution of the binderresin and presence of residual magnetic flux in the powder even afterdemagnetization although the intensity is low, as a result of themolding conducted on the mixture at the room temperature by using a slidepoxy resin.

SEM observation of the molded bodies showed non-uniform distribution ofthe binder resin, allowing mixing of discrete magnet powder and bindingresin. Many pores were also observed.

Sample No. 3a were defective as stated above, and could not effectivelybe subjected to measurement of characteristics such as mechanicalstrength.

EXAMPLE 4

The following two types of magnet powder, binder resin (thermoplasticresin) and additive were mixed together to form a mixture which wassubjected to kneading, and the kneaded matter was granulated (grained)into granules. The granular material was charged in a mold of a moldingmachine and was pressure-molded (warm molding) under influence of amagnetic field. After the molding, cooling was executed whilemaintaining the same level of pressure as the molding pressure, wherebySample Nos. 23a to 31a of rare earth bonded magnets were obtained. Theamounts of the matters shown below are expressed in terms of thecontents in the kneaded composition.

Composition

Sm--Co type magnet powder:

Sm(Co₀.0672 Fe₀.22 Cu₀.08 Zr₀.028)₈.35

70.5 wt % (content in product magnet is almost the same as this value)

Sm--Fe--N type magnet powder:

Nd₁₂.0 Fe₇₇.8 Co₄.3 B₅.9

23.5 wt %

Thermoplastic resin:

Polyamide resin (nylon 12)

5.0 wt %

Antioxidant:Phenol-type antioxidant 1.0 wt %

Mixing: Mixed by means of Henschel mixer

Kneading: Kneaded by a twin-screw kneader at 150° C. to 300° C. Screwspeed 100 to 300 r.p.m., kneading time (time of stay in the kneader) 5to 15 minutes

Granulation (Graining):

Kneaded composition was granulated into granules of average sizes shownin Table 9, through grinding and classification.

Molding:

Granulated material was charged into a mold at room temperature bygrading method, and pressure-molded under the influence of a transversemagnetic field (15 KOe) when temperature has been raised to 220° C.(first temperature). The molding pressure was 10 kgf/mm².

Cooling:

The molded body was cooled down to a predetermined pressure releasingtemperature of 100° C. (second temperature) while the pressure wasmaintained. Samples were then taken out from the molds. The cooling wasconducted by water cooling. The cooling rate in the cooling underpressure was 20 C./sec.

Product shapes:

Flat tabular shape (15 mm wide×2.5 mm thick and 5 m m high, aligned inthe heightwise direction)

The weight, density, porosity, and height were examined with the samplesof the rare earth bonded magnet thus obtained. The results are shown inTable 9.

From Table 9, it will be seen that superior quantitative control can beachieved by virtue of the suitable selection of granule size, thusenabling production of bonded magnet having low porosity and highdimensional precision. In particular, when the granule size falls withinthe range of from 0.01 mm to 2 mm as are the cases of Sample Nos. 23a to30a, both low porosity (1.5 wt % or less, in particular 1% or less) andhigh dimensional precision (dimensional error being within ±5/100 mm)could be simultaneously obtained.

EXAMPLE 5

The following type of magnet powder, binder resin (thermoplastic resin)and additive were mixed together to form a mixture which was subjectedto kneading, and the kneaded matter was -granulated (grained) intogranules. The granular material was charged in a mold of a moldingmachine and was pressure-molded (warm molding) under influence of amagnetic field. After the molding, cooling was executed whilemaintaining the same level of pressure as the molding pressure, wherebySample Nos. 32a to 42a of rare earth bonded magnets were obtained. Theamounts of the matters shown below are expressed in terms of thecontents in the kneaded composition.

Composition

Nd--Fe--B type magnet powder:

Nd₁₂.6 Fe₆₉.3 Co₁₂.0 B₆.0 Zr₀.1

97.0 wt % (content in product magnet is almost the same as this value)

Thermoplastic resin:

Resin A or F shown in Table 1

1.5 wt % each

Antioxidant: Hydrazine-type antioxidant 1.4 wt %

Lubricant: Zinc stearate 0.1 wt %

Mixing: Mixed by means of Henschel mixer

Kneading: Kneaded by a twin-screw kneader at 150° C. to 350° C. Screwspeed 100 to 300 r.p.m., kneading time (time of stay in the kneader) 5to 10 minutes

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 0.3mm through grinding and classification.

Molding:

Granulated material was charged into a mold at room temperature, andpressure-molded under the influence of a radial magnetic field (15 KOe)when temperature has been raised to molding temperature shown in Table10 (first temperature). The molding pressure was 15 kgf/mm².

Cooling:

The molded body was cooled down to a predetermined pressure releasingtemperature of 100° C. (second temperature) while the pressure wasmaintained. The pressure was released after demagnetization, followed bycooling down to normal temperature. Samples were then taken out from themolds. The cooling was conducted by water cooling. The cooling rate inthe cooling under pressure was 30° C./sec.

Product shapes:

Cylindrical shape (20 mm outside diameter×18 mm inside diameter×5 mmheight, compacted in the heightwise direction)

Flat tabular shape (20 mm square×3 mm thick (for use as mechanicalstrength test piece)

Magnetic performance (maximum magnetic energy product (BH)max), as wellas density, porosity and mechanical strength were examined with thesamples of the rare earth magnet powder thus obtained. The results areshown in Table 10. Methods of evaluations of the items are the same asthose in Example 1.

As shown in Table 10, the binder resin was softened or molten when themolding temperature was set to be not lower than the thermal deformationtemperature of the binder resin as are the cases of Sample Nos. 32a to42a, so that the molding could be successfully carried out.

In particular, further reduction in porosity and, hence, furtherimprovement in the magnetic performance cold be achieved when themolding was conducted at a temperature not lower than the meltingtemperature of the binder resin, as are the cases of Sample Nos. 33a to36a and 40a to 42a.

EXAMPLE 6

The following type of magnet powder, binder resin (thermoplastic resin)and additive were mixed together to form a mixture which was subjectedto kneading, and the kneaded matter was granulated (grained) intogranules. The granular material was charged in a mold of a moldingmachine and was pressure-molded (warm molding) without application ofmagnetic field. After the molding, cooling was executed whilemaintaining the same level of pressure as the molding pressure, wherebySample Nos. 43a to 52a of rare earth bonded magnets were obtained. Theamounts of the matters shown below are expressed in terms of thecontents in the kneaded composition.

Composition

Nano-crystalline Nd--Fe--B type magnet powder:

Nd₅.5 Fe₆₆ B₁₈.5 Co₅ Cr₅

98.0 wt % (content in product magnet is almost the same as this value)

Thermoplastic resin:

Resin A or G shown in Table 1

1.0 wt % each

Antioxidant: Hydrazine-type antioxidant 1.0 wt %

Lubricant: Zinc stearate 0.1 wt %

Mixing: Mixed by means of Henschel mixer

Kneading:

Kneaded by a twin-screw kneader at 150° C. to 350° C. Screw speed 100 to300 r.p.m., kneading time (time of stay in the kneader) 10 to 15 minutes

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 0.1mm through grinding and classification.

Molding:

Granulated material was charged into a mold and pressure-molded whentemperature has been raised to a predetermined molding temperature. Morespecifically, the molding temperature was 200° C. for the resin A and300° C. for the resin G. The molding pressure was 25 kgf/mm².

Cooling:

The molded body was cooled down to a predetermined pressure releasingtemperature shown in Table 11 (second temperature) while the pressurewas maintained. Samples were then taken out from the molds. The coolingwas conducted by water cooling. The cooling rate in the cooling underpressure was 50° C./sec.

Product shapes:

Cylindrical shape (10 mm outside diameter×7 mm inside diameter×7 mmheight, compacted in the heightwise direction)

Magnetic performance (maximum magnetic energy product (BH)max), as wellas density, porosity and the outside diameter were examined with thesamples of the rare earth magnet powder thus obtained. The results areshown in Table 11. Methods of evaluations of the items are the same asthose in Example 1.

As demonstrated by Sample Nos. 43a to 52a shown in Table 11, productmagnets exhibit low porosity, high density and high dimensionalprecision (dimensional error within +5/100 mm), as well as superiormagnetic performance, when the pressure releasing temperature is nothigher than the melting temperature of the binder resin or when thedifference between the pressure releasing temperature and the moldingtemperature is not smaller than 20° C. These advantages are moreremarkable when lower pressure releasing temperature is employed.

In particular, when the pressure releasing temperature is not higherthan the thermal deformation temperature of the binder resin as are thecases of Sample Nos. 46a, 47a, 50a, 51a and 52a, it is possible toobtain high density which approximates stoichiometric density, so thatthe product magnet exhibits extremely superior performance by makingfull use of the characteristics of the magnet powder.

EXAMPLE 7

The following types of magnet powder, binder resin (thermoplastic resin)and additive were mixed together to form a mixture which was subjectedto kneading, and the kneaded matter was granulated (grained) intogranules. The granular material was charged in a mold of a moldingmachine at the room temperature and was pressure-molded (warm molding)without application of magnetic field. After the molding, cooling wasexecuted while maintaining the same level of pressure as the moldingpressure, whereby Sample Nos. 1b to 6b of rare earth bonded magnets wereobtained in which the magnet powder particles were bonded together bysolidified binder resin.

Nd--Fe--B type magnet powder: Nd₁₂.0 Fe₇₇.8 Co₄.3 B₅.9

97 wt % (content in product magnet is almost the same as this value)

Polyamide resin (PA12):

Melting temperature 178° C., thermal deformation temperature 145° C.

1.6 wt %

Antioxidant: Hydrazine-type antioxidant 1.0 wt %

Mixing: Mixed by means of Henschel mixer

Kneading: Kneaded by a twin-screw kneader at 150° C. to 250° C. Screwspeed 100 to 250 r.p.m.,

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 1 mmthrough grinding.

Molding:

The composition (granulated material) was charged into a mold andpressure-molded by means of a punch when temperature has been raised toa temperature shown in Table 12 (first temperature). The moldingpressure was 10 kgf/mm².

Cooling:

The molded body was cooled while the pressure was maintained. Thepressure was released when the temperature has come down to 100° C.(second temperature). Samples were then taken out from the molds. Thecooling was conducted by air cooling. The cooling rate in the coolingunder pressure was 0.5° C./sec.

Product shapes:

Solid cylindrical shape (10 mm diameter×7 mm height)

Flat tabular shape (20 mm square×3.0 mm thick)

Magnetic performance (magnetic flux density Br, coercive force iHc,maximum magnetic energy product (BH)max), as well as density, porosityand mechanical strength were examined with the samples of the rare earthmagnet powder thus obtained. The results are shown in Table 12. Methodsof evaluations of the items appearing in Table 12 are the same as thosein Example 1.

As will be seen from Table 12, the density and porosity of the productbonded magnet tend to increase and decrease, respectively, when themolding temperature is elevated. Specifically low porosity is obtainedso as to provide high levels of mechanical strength and magneticperformance, when the molding temperature is not lower than the meltingtemperature of the thermoplastic resin, as in the cases of Sample Nos.3b to 6b.

EXAMPLE 8

Each of the magnet powders shown in Table 13, a binder resin(thermoplastic resin) and additive were mixed together to form a mixturewhich was subjected to kneading, and the kneaded matter was granulated(grained) into granules. The granular material was charged in a mold ofa molding machine and was pressure-molded (warm molding) withoutapplication of magnetic field. After the molding, cooling was executedwhile maintaining the same level of pressure as the molding pressure,whereby Sample Nos. 7b to 31b of rare earth bonded magnets were obtainedin which the magnet powder particles were bonded together by solidifiedbinder resin.

Composition

Magnetic powder:

Each of the powders 1 to 5 shown in Table 13

96.0 wt % (content in product magnet is almost the same as this value)

Polyamide resin (PA 12):

Melting temperature 178° C., thermal deformation temperature 145° C.

2.55 wt %

Antioxidant: Hydrazine-type antioxidant, 1.4 wt %

Other additive: Higher fatty acid (Stearic acid), 0.05 wt %

Mixing: Mixed by means of Henschel mixer

Kneading: Kneaded by a twin-screw kneader at 150° C. to 250° C. Screwspeed 100 to 250 r.p.m.,

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 0.5mm through grinding and classification.

Molding:

The composition (granulated material) was charged into a mold andpressure-molded by means of a punch when temperature has been raised to230° C. (first temperature). The molding pressure was 15 kgf/mm².

Cooling:

The molded body was cooled while the pressure was maintained. Thepressure was released when the molded body has been cooled totemperatures shown in Table 14 (second temperature). Samples were thentaken out from the molds. The cooling was conducted by air cooling. Thecooling rate in the cooling under pressure was 2° C./sec.

Product shapes:

Cylindrical shape (20 mm outside diameter×18 mm inside diameter×10 mmheight)

The values of the average granule sizes appearing in Table 13 wereobtained through measurement in accordance with F.S.S.S. (FisherSub-Sieve Sizer). The density, porosity and circularity (dimensionalprecision) were examined with the samples of the rare earth magnetpowder thus obtained. The results are shown in Tables 14 and 15. Methodsof evaluations of the items appearing in Tables 14 and 15 were asfollows.

Density: The same as that in Example 1

Porosity: The same as that in Example 1

Circularity:

Outside diameter of the molded body was measured at 10 points and thecircularity was determined through the following calculation based onthe maximum and minimum measured outside diameters.

Circularity=(maximum value-minimum value)/2

As will be seen Tables 14 and 15, such a tendency was observed with eachof the magnet powder compositions that a lower pressure releasingtemperature in the course of cooling provides a greater density andsmaller porosity of the bonded magnet and reduces the risk ofdeformation of the sample when the same is separated from the mold, thusoffering improved circularity (dimensional precision). Extremely highdegree of circularity (dimensional precision) is obtained when thepressure releasing temperature is not higher than the meltingtemperature (178° C.), in particular not higher than the thermaldeformation temperature (145° C.) of the thermoplastic resin used as thebinder resin.

EXAMPLE 9

The following type of magnet powder, each of the binder resins(thermoplastic resins) shown in Table 16 and additive were mixedtogether to form a mixture which was subjected to kneading, and thekneaded matter was granulated (grained) into granules. The granularmaterial was charged in a mold of a molding machine and waspressure-molded (warm molding) under the influence of a magnetic field.After the moBding, coong w exeted ile intaing e sa lev of essu as emoing essu, wheby mpleos.3 to b ofare rth ndedagne werobtaed iwhicthegnetowdepartles re bded geth by lidied bder sin.

Magnet powder: Sm(Co₀.672 Fe₀.22 Cu₀.08 Zr₀.028)₈.35

96.5 wt % (content in product magnet is almost the same as this value)

Thermoplastic resin:

One of resins A to G shown in Table 16, A+B, each 2.3 wt %

Antioxidant: Phenol-type antioxidant 1.2 wt %

Mixing: Mixed by means of Henschel mixer

Kneading:

Kneaded by a twin-screw kneader. As to the kneading temperature,reference be made to Table 5. Screw speed 100 to 250 r.p.m.,

Granulation (Graining):

Kneaded composition was granulated into granules of average size of 0.5mm through grinding and classification.

Molding:

The composition (granulated material) was charged into a mold at theroom temperature and pressure-molded by means of a punch when the moldtemperature has been raised to a temperature shown in Table 16 (firsttemperature). The molding pressure was 10 kgf/mm². A radial magneticfield (aligning magnetic field of 15 KOe) was applied immediately beforethe pressurizing.

Cooling:

The molded body was cooled while the pressure was maintained.

Demagnetization was conducted at a temperature shown in Tables 17 to 19(second temperature). Samples were then taken out from the molds. Thecooling was conducted by water cooling. The cooling rate in the coolingunder pressure was 10° C./sec.

Product shapes:

Cylindrical shape (30 mm outside diameter×27 mm inside diameter×5 mmheight)

The values of thermal deformation temperature appearing in Table 16 wereobtained through measurement in accordance with the aforementioned ASTMD648.

The density, porosity and the circularity (dimensional precision) of thesamples of rare earth bonded magnet thus obtained were measured toobtain the results as shown in Tables 17, 18 and 19.

Evaluations of the measurement items appearing in Tables 17, 18 and 19were the same as those in Examples 1 and 8.

As will be seen Tables 17 to 19, such a tendency was observed with eachof the binder resin compositions that a lower pressure releasingtemperature in the course of cooling provides a greater density andsmaller porosity of the bonded magnet, thus offering improvedcircularity (dimensional precision). Extremely high degree ofcircularity (dimensional precision) is obtained when the pressurereleasing temperature is not higher than the melting temperature, inparticular not higher than the thermal deformation temperature of thethermoplastic resin used as the binder resin.

Magnetic performance was examined with each of the samples shown inTables 17 to 19. All these samples showed superior magneticcharacteristics: namely, magnetic flux densities Br not lower than 7.0KG, coercive force iHc not smaller than 7 KOe and maximum magneticenergy product (BH)max not smaller than 13 MGOe.

EXAMPLE 10

Magnet powders, binder resins (thermoplastic resins) and an additivewere mixed to form compositions as shown in Table 20. The mixture waskneaded and the kneaded composition was granulated (grained) to formgranules which were then charged in a mold of a molding machine andpressure-molded (warm molding) without application of magnetic field.Then, cooling was conducted in which the pressure of the same level asthe molding pressure was maintained until the temperature comes down tothe melting point and thereafter the cooling was continued withprogressive reduction of the pressure to about 50% the compactingpressure, whereby Sample Nos. 63b to 80b of the rare earth bonded magnetwere obtained in which magnet powder particles were bonded together bythe solidified binder resin. The contents of the magnetic powder in thesample magnets thus obtained were almost the same as those in Table 20.

Magnet powder: Nd₁₂.0 Fe₇₇.8 Co₄.3 B₅.9

Polyamide resin (PA12):

Melting temperature 178° C.

Thermal deformation temperature 145° C.

Antioxidant: Hydrazine-type antioxidant

Mixing: Mixed by means of a Henschel mixer

Kneading: Kneaded by a twin-screw kneader,

Kneading temperature 100 to 250° C.

Screw speed 100 to 250 r.p.m.

Granulation (Graining):

Kneaded composition was granulated into granules of an average size of0.5 mm through grinding and kneading.

Molding:

The composition (granules) was charged into a mold at room temperatureand pressure-molded by means of a punch when the mold has been heated to220° C. (first temperature). The molding pressure was 20 kgf/mm².

Cooling:

Cooling was conducted while keeping the molded body under pressure(pressure progressively decreased in relation to time), and the pressurewas completely nullified when the temperature has come down to thetemperatures shown in Tables 21 and 22 (second temperature). The sampleswere then taken out of the mold. The cooling was conducted by watercooling. The rate of cooling under pressure was 50° C./sec.

Product shape:

Roof-tile shape (8 mm outside curvature radius (R)×7 mm inside curvatureradius (r)×120 sector angle×8 mm height)

The density, porosity, circularity (dimensional precision) and themagnetic characteristic (maximum magnetic energy product (BH)max) weremeasured on each of these samples to obtain the results as shown inTables 21 and 22.

Methods of evaluation of the items appearing in tables 21 and 22 werethe same as those in Examples 1 and 8.

As will be seen Tables 21 and 22, such a tendency was observed with eachof the magnet powder contents that a lower pressure releasingtemperature in the course of cooling provides a greater density andsmaller porosity of the bonded magnet, thus offering improvedcircularity (dimensional precision). Extremely high degree ofcircularity (dimensional precision) is obtained when the pressurereleasing temperature is not higher than the melting temperature, inparticular not higher than the thermal deformation temperature of thethermoplastic resin used as the binder resin.

EXAMPLE 11

Magnet powders, binder resins (thermoplastic resins) and additives suchas antioxidants were mixed to form compositions shown in Table 23. Eachmixture was kneaded and the kneaded composition was granulated (grained)to form granules which were then charged in a mold of a molding machineat room temperature and pressure-molded (warm molding) with or withoutapplication of magnetic field. Then, cooling was conducted to obtainSample Nos. 1c to 6c of the rare earth bonded magnet.

The conditions under which these samples were prepared were as follows.

Mixing: Mixed by means of a Henschel mixer

Kneading:

Kneaded by a twin-screw kneader. Kneading temperature was 200 to 350° C.

Screw speed was 100 to 300 r.p.m. The kneaded composition was extrudedin the form of a circular rod of 10 mm diameter which was then cut intopellets of 5 to 15 mm long.

Granulation:

The pellets were ground into powder particles of 1 mm mesh or smallersize. Mean particle size was from 0.2 to 0.8 mm in each sample.

Molding:

Molded by means of a hydraulic press. As to the molding temperature, areference be made to Table 24. The molding pressure was 10 kgf/mm². Theproduct shape was cylindrical (20 mm outside diameter×17 mm insidediameter×5 m m height)

Cooling:

Cooled down to normal temperature under the atmospheric pressure.

The magnetic flux density Br, coercive force iHc, maximum magneticenergy product (BH)max, density and porosity were measured on thesesamples of the rare earth bonded magnet thus obtained. The results areshown in Table 24. The measurements were carried out in accordance withthe following methods.

Magnetic performance:

Specimen of 5 mm square was cut out of each sample and pulse magnetizedat 40 KOe. The magnetized test piece was then subjected to measurementconducted by using a vibration specimen magnetometer (VSM).

Measurement of density:

The same as that in Example 1

Measurement of porosity:

The same as that in Example 1

COMPARATIVE EXAMPLE 3

Magnet powders, binder resins (thermoplastic resins) and additives suchas antioxidants were mixed to form compositions shown in Table 23. Eachmixture was kneaded and the kneaded composition was pelletized intopellets which were then charged in a mold of a molding machine at roomtemperature and pressure-molded (low-temperature warm molding) with orwithout application of magnetic field. Then, cooling was conducted toobtain Sample Nos. 7c to 12c of the rare earth bonded magnet.

The conditions under which these samples were prepared were as follows.

Mixing: The same as that in Example 11

Kneading: The same as that in Example 11

Granulation: Not conducted

Molding:

The same as that in Example 11 except that different moldingtemperatures and pressures were employed as shown in Table 25.

Cooling:

Cooled under pressure was conducted down to the pressure releasingtemperatures shown in Table 25. The pressure was released afterdemagnetization of the molded body in the mold.

The results of the molding operations were as follows.

Preparation of Sample Nos. 7c to 12c faced a difficulty in charging thepellets into the mold, since the minimum mold gap size (magnet wallthickness) was as small as 1.5 mm while the pellet size was 10 mm×5 mmat the smallest. Therefore, the molding required grinding the pellets bya punch, or heating and melting the pellets, followed by a plurality ofstroke cycles of the punch for forcibly charging the material. Thus, alaborious work was required in the molding, resulting in a prolongedcycle time, making it difficult to mold the magnet at low costs. Inaddition, it was difficult to control the amount of the material chargedinto the mold, so that the dimensions of the products tend to largelyfluctuate with respect to the target dimensions (sample length), thusimpairing dimensional stability.

In addition, each of Sample Nos. 7c to 12c showed high level of porositydespite elevated molding pressure, due to the fact that the moldingtemperature was below the thermal deformation temperature of the binderresin.

EXAMPLE 12

The following types of magnet powder, binder resin (thermoplastic resin)and additive were mixed together to form a mixture which was subjectedto kneading, and the kneaded matter was granulated (grained) intogranules. The granular material was charged in a mold of a moldingmachine at the room temperature and was pressure-molded (cold molding)under the influence of a magnetic field. Then, a heat treatment wasconducted followed by cooling, whereby Sample No. 13c of rare earthbonded magnets was obtained.

Magnet powder:

Nd₉.6 Pr₂.4 Fe₇₇.8 Co₄.3 B₅.9

Average particle size 20 μm (measured by F.S.S.S.)

96.5 wt % (content in product magnet is almost the same as this value)

Binder resin:

PPS (Polyphenylene sulfide) resin 2.3 wt %

Antioxidant: Hydrazine-type antioxidant 1.2 wt %

Mixing: The same as that in Example 11

Kneading: The same as that in Example 11

Granulation:

Pellets were ground into powder particles of 0.8 mm mesh or smallersize.

Average granule size was 0.5 mm.

Molding:

Molding was conducted by using a hydraulic press at the room temperatureand molding pressure of 70 kgf/mm² without application of magneticfield. Tentative molded body was obtained.

The shape of the molded body was cylindrical (target dimensions: 25.00mm outside diameter×23.00 mm inside diameter×1000 mm height).

Mold size: Annular gap of 24.35 mm outside diameter×22.40 mm insidediameter

Heat treatment:

Heated at 320° C. for 10 minutes to melt the resin component therebybonding resin particles together to improve mechanical strength.

The dimensional precision, magnetic performance, density and porositywere examined on this Sample of the rare earth bonded magnet thusobtained. The results are shown below.

Dimensions of molded body:

24.98 mm outside diameter 23.01 mm inside diameter×10.02 mm height

(average value over n=10)

Fluctuation of height (length)=0.02

Magnetic performance:

Br=6.8 KG, iHc=9.63 KOe,

(BH)max=9.6 MGOe

Density: 6.8 g/cm³

Porosity: 2.06%

(The magnetic performance was measured by means of a D.C. magnetometerunder application of maximum magnetic field of 25 KOe, afterpulse-magnetization at 40 KOe.)

In Example 12, it was confirmed that the charging (feeding) of thematerial into the mold could be delicately controlled, so that anextremely high dimensional precision could be obtained. At the sametime, superior magnetic performance could be obtained, as well asextremely small porosity.

COMPARATIVE EXAMPLE 4

The following types of magnet powder, binder resin (thermoplastic resin)and additive were mixed together to form a mixture which was subjectedto kneading, and pellets of the kneaded composition was charged in amold of a molding machine at the room temperature and waspressure-molded (cold molding) under the influence of a magnetic field.Then, a heat treatment was conducted followed by cooling, whereby SampleNo. 14c of rare earth bonded magnet was obtained.

Magnet powder:

Nd₉.6 Pr₂.4 Fe₇₇.8 Co₄.3 B₅.9

Average particle size 20 pm (measured by F.S.S.S.)

96.5 wt % (content in product magnet is almost the same as this value)

Binder resin:

PPS (Polyphenylene sulfide) resin 2.3 wt %

Antioxidant: Hydrazine-type antioxidant 1.2 wt %

Mixing: The same as that in Example 11

Kneading: The same as that in Example 11

Granulation: Not conducted.

Molding: The same as that in Example 12

Heat treatment: The same as that in Example 12

The dimensional precision was examined on this Sample of the rare earthbonded magnet thus obtained. The results are shown below.

Dimensions of molded body:

24.98 mm outside diameter×2 3.01 mm inside diameter×9.52 mm height

(average value over n=10)

Fluctuation of height (length)=0.90

Comparative Example 4 showed that, due to lack of the granulating step,feed of the material into the mold was rendered unstable as in the caseof Comparative Example 3. Thus, it was found difficult to stably carryout the molding due to fluctuation particularly in the heightwisedimension. Furthermore, the punch had to be moved up and down repeatedlyin order to feed the material so as to crush and force the material intothe mold, resulting in a prolonged molding time. In addition, the punchwas broken during feeding of the material.

EXAMPLE 13

The following types of magnet powder, binder resin (thermosetting resin)and additive were mixed together to form a mixture which was subjectedto kneading, and the kneaded matter was granulated (grained) intogranules. The granular material was charged in a mold of a moldingmachine at the room temperature and was pressure-molded (warm molding)under the influence of a magnetic field. Then, a heat treatment wasconducted to harden the resin, followed by cooling, whereby Sample No.15c of rare earth bonded magnets was obtained.

Magnet powder:

Sm(Co₀.672 Fe₀.22 Cu₀.08 Zr₀.028)₈.35

Average particle size 20 μm (measured by F.S.S.S.)

97.5 wt % (content in product magnet is almost the same as this value)

Binder resin:

Phenol resin 2.45 wt %+organic solvent

Solid phase at room temperature, Softening temperature 60° C.

Antioxidant: Zinc stearate 0.05 wt %

Mixing:

Mixing was conducted by using a general purpose blender while allowingthe organic solvent to evaporate.

Granulation:

Granulated into powder of 180 μm mesh or smaller size, by grinding.Average granule size was 100 μm.

Molding:

Molding was conducted by using a hydraulic press at 80° C. and moldingpressure of 30 kgf/mm² under the influence of an aligning magnetic fieldof 17 KOe (transverse magnetic field). Tentative molded body wasobtained.

The shape of the molded body was roof-tile shape (target dimensions:15.00 mm outside curvature radius (R)×12.00 mm inside curvature radius(r)×120° sector angle×8.00 mm height).

Magnetic field aligned in the radial direction. Demagnetized in the moldafter the pressurizing.

Mold size: Gap of R 14.70 mm×r 11.80 mm×120° sector angle

Heat treatment:

Heated at 180° C. for 2 hours.

The dimensional precision, magnetic performance, density and porositywere examined on this Sample of the rare earth bonded magnet thusobtained. The results are shown below.

Dimensions of molded body:

R 15.03 mm×r 12.01 mm×120° sector angle×17.98 mm (average value overn=10)

Fluctuation of height (length)=0.015

Magnetic performance:

Br=7.8 KG, iHc=7.10 KOe,

(BH)max=13.6 MGOe

Density: 7.11 g/cm³

Porosity: 4.63%

(The magnetic performance was measured by VSM on a specimen of 5 mmsquare specimen cut out of the roof-tile shaped molded body.)

In Example 13, it was confirmed that the charging (feeding) of thematerial into the mold could be delicately controlled, so that anextremely high dimensional precision could be obtained. At the sametime, superior magnetic performance could be obtained, as well asextremely small porosity.

COMPARATIVE EXAMPLE 5

The following types of magnet powder, binder resin (thermosetting resin)and additive were mixed together to form a mixture which was charged ina mold of a molding machine at the room temperature and waspressure-molded (cold to low-temperature warm molding) under theinfluence of a magnetic field so that the resin was hardened. Then, aheat treatment was conducted followed by cooling, whereby Sample Nos.16c, 17c and 18c of rare earth bonded magnets were obtained.

Magnet powder:

Sm(Co₀.672 Fe₀.22 Cu₀.08 Zr₀.028)₈.35

Average particle size 20 μm (measured by F.S.S.S.)

97.5 wt %

Binder resin:

Phenol resin 2.45 wt %+organic solvent

Solid phase at room temperature, Softening temperature 60° C.

Antioxidant: Zinc stearate 0.05 wt %

Mixing:

Mixing was conducted by using a general purpose blender while allowingthe organic solvent to evaporate.

Molding:

Molding was conducted by using a hydraulic press at 25° C. (Sample No.16c), 40° C. (Sample No. 17c) and 50° C. (Sample No. 18c), and atmolding pressure of 30 kgf/mm² under the influence of an aligningmagnetic field of 17 KOe (transverse magnetic field). Tentative moldedbody was obtained. The molded body was demagnetized while it is in themold and then taken out of the mold.

The shape of the molded body was roof-tile shape (target dimensions:15.00 mm outside curvature radius (R)×1 2.00 mm inside curvature radius(r)×120° sector angle×8.00 mm height).

Magnetic field aligned in the radial direction.

Mold size: Gap of R 14.70 mm×r 11.80 mm×120° sector angle

Heat treatment:

Heated at 180° C. for 2 hours.

The dimensional precision of Samples of the rare earth bonded magnetthus obtained were examined. The results are shown below.

Dimensions of molded body:

R 15.01 mm×r 11.98 mm×120° sector angle×17.46 mm (average value overn=10)

Fluctuation of height (length)=0.58

In Comparative Example 5 (Sample Nos. 16c to 18c), the feed of thematerial into the mold was rendered unstable due to lack of thegranulating step. Thus, it was difficult to stably conduct the moldingdue to fluctuation particularly in the heightwise dimension. Inaddition, the punch had to be moved up and down repeatedly to feed thematerial, thus prolonging the molding time. Furthermore, an additionalstep had to be employed for cleaning the punch, in order to remove thematerial depositing and remaining on side faces of the punch withoutbeing fed into the mold.

EXAMPLE 14

The following types of magnet powder and binder resin (thermosettingresin) were mixed together to form a mixture which was subjected tokneading, and the kneaded matter was granulated (grained) into granules.The granular material was charged in a mold of a molding machine at theroom temperature and was pressure-molded (cold molding) withoutapplication of magnetic field. Then, a heat treatment was conducted toharden the resin, followed by cooling, whereby Sample No. 19c of rareearth bonded magnets was obtained.

Magnet powder:

Nd₁₂.0 Fe₇₇.8 Co₄.3 B₅.9

Average particle size 25 μm (measured by F.S.S.S.)

98.0 wt % (content in product magnet is almost the same as this value)

Binder resin:

Bisphenol A novolak resin+amine-type curing agent

2.0 wt % in total

Liquid phase at room temperature

Mixing:

Mixing was conducted by using a general purpose blender.

Kneading:

Kneading was conducted by using a Raikai kneader.

Granulation:

Kneaded composition was formed into granules of 0.6 mm mesh or smallersize by an extrusion granulator.

Average granule size was 0.4 mm.

Molding:

Molding was conducted by using a hydraulic press at the room temperatureand molding pressure of 50 kgf/mm². Tentative molded body was obtained.

The shape of the molded body was cylindrical (target dimensions: 10.00mm outside diameter×8.00 mm inside diameter×7.00 mm height).

Magnetic field aligned in the radial

Mold size: Gap of 9.65 mm outside diameter×7.75 mm inside diameter

Heat treatment:

Heated at 150° C. for 1 hour.

The dimensional precision, magnetic performance, density and porositywere examined on this Sample of the rare earth bonded magnet thusobtained. The results are shown below.

Dimensions of molded body:

9.99 mm outside diameter×7.98 mm inside diameter×7.02 mm

height(average value over n=10)

Fluctuation of height (length)=0.030

Magnetic performance:

Br=7.51 KG, iHc=9.67 KOe,

(BH)max=11.5 MGOe

Density: 6.44 9/cm³

Porosity: 5.40%

(The magnetic performance was measured by a D.C. magnetometer underapplication of maximum magnetic field of 25 KOe, afterpulse-magnetization at 40 KOe.)

In Example 14, it was confirmed that the charging (feeding) of thematerial into the mold could be delicately controlled, so that anextremely high dimensional precision could be obtained. At the sametime, superior magnetic performance could be obtained, as well as smallporosity.

COMPARATIVE EXAMPLE 6

The same magnet powder and binder resin (thermosetting resin) as thoseused in Example 14 were mixed together to form a mixture which wascharged in a mold of a molding machine and was pressure-molded (coldmolding) without application of magnetic field. Then, a heat treatmentwas conducted to harden the resin, followed by cooling, whereby SampleNo. 20c of rare earth bonded magnets was obtained.

Thus, the process was the same as Example 14 except that granulation wasomitted.

The dimensional precision of Sample of the rare earth bonded magnet thusobtained was examined. The results are shown below.

Dimensions of molded body:

9.99 mm outside diameter×7.98 mm inside diameter×6.72 mm height

(average value over n=10)

Fluctuation of height=0.86

In Comparative Example 6, the feed of the material into the mold wasrendered unstable due to lack of the granulating step. Thus, it wasdifficult to stably conduct the molding due to fluctuation particularlyin the heightwise dimension. In addition, the punch had to be moved upand down repeatedly to feed the material, thus prolonging the moldingtime. Furthermore, an additional step had to be employed for cleaningthe punch, in order to remove the material depositing and remaining onside faces of the punch without being fed into the mold.

EXAMPLE 15

The following types of magnet powder, binder resin (thermoplastic resin)and additive were mixed together to form a mixture which was subjectedto kneading, and the kneaded matter was granulated (grained) intogranules. The granular material was charged in a mold of a moldingmachine at the room temperature and was pressure-molded (warm molding)without application of magnetic field followed by cooling, wherebySample Nos. 21c to 27c of rare earth bonded magnet were obtained.

Magnet powder:

Nd₁₂.0 Fe₇₇.8 Co₄.3 B₅.9

Average particle size 25 μm (measured by F.S.S.S.)

97.2 wt % (content in product magnet is almost the same as this value)

Binder resin:

Polyamide (PA12) resin 1.4 wt %

Antioxidant: Hydrazine-type antioxidant 1.4 wt %

Mixing: The same as that in Example 11

Kneading:

Kneading was conducted by using a twin-screw kneader, at kneadingtemperatures of 150 to 250° C. and screw speed of 100 to 250 r.p.m.

The kneaded composition was extruded in the form of a circular rod of 10mm diameter and was pelletized into pellets of 5 to 15 mm long.

Granulation:

Grain size was regulated through grinding and classification. As tomaximum grain size and minimum grain size, reference be made to Table26.

Molding:

Molding was conducted by using a hydraulic press at a moldingtemperature of 230° C. and molding pressure of 10 kgf/mm².

The shape of the molded body was cylindrical (target dimensions: 18.00mm outside diameter×14.00 mm inside diameter×1000 mm height).

Charging of material into the mold was conducted weighing 6.44 g of thegranular material and feeding the whole weighed material into the moldgap in the circumferential direction.

The density, porosity and the height were examined on these Sample ofthe rare earth bonded magnet thus obtained. The results are as shown inTable 26.

As will be seen from Table 26, the porosity tends to increase as thegranule size of the granulated material becomes smaller. This isattributed to the fact that smaller granule size enhances trapping ofair during the molding.

Therefore, the lower limit of the granule size is preferably set to 0.1mm, more preferably to 0.02 mm and most preferably to 0.05 mm.

Feeding of the material into the mold can be conducted withoutdifficulty if the maximum granule size of the granulated material is notgreater than the minimum mold gap size (2.0 mm). Each sample showed adimensional error of ±5/100 mm, thus demonstrating high dimensionalprecision.

EXAMPLE 16

The following types of magnet powder, binder resin (thermoplastic resin)and additive were mixed together to form a mixture which was subjectedto kneading, and the kneaded matter was granulated (grained) intogranules. The granular material was charged in a mold of a moldingmachine at the room temperature and was pressure-molded (warm molding)without application of magnetic field followed by cooling, wherebySample Nos. 28c to 31c of rare earth bonded magnet were obtained.

Magnet powder:

Nd₁₂.0 Fe₇₇.8 Co₄.3 B₅.9

Average particle size 25 μm (measured by F.S.S.S.)

97.0 wt % (content in product magnet is almost the same as this value)

Binder resin:

Polyamide (PA12) resin+polyamide (PA6-12) resin, PA12/PA6-12 ratio being5/5, total content 1.6 wt %

Antioxidant: Hydrazine-type antioxidant 1.4 wt %

Mixing: The same as that in Example 11

Kneading: The same as that in Example 15

Granulation:

Granulation was done by grinding and classification such that themaximum granule size of 5 mm (average size 2 mm).

Molding:

Molding was conducted by using a hydraulic press at a moldingtemperature of 230° C. and molding pressure of 10 kgf/mm².

The shape of the molded body was flat tabular shape. (target dimensions:10.00 mm wide×(a) mm thick ((a) being variable)×10.00 mm height).

A plurality of molds having different sizes "a" (shown in Table 27) withfixed width 10 mm were used. The amount of granular material chargedinto the molds was adjusted (as shown in Table 27), such that the moldedbody has the height of 10 mm regardless of variation of the thicknesssize "a".

Weights and heights of the samples of rare earth bonded magnet thusobtained were measured to obtain the results shown in Table 27.

As will be seen from table 27, the dimensional error is smaller when themaximum granule size of the granulated material becomes smaller relativeto the minimum mold gap size "a". In particular, specifically highdimensional precision is obtained when the mold gap size "a" is notsmaller than the maximum granule size (5 mm), as in the cases of SampleNos. 31c to 33c.

EXAMPLE 17

Sample Nos. 34c to 40c of the rare earth bonded magnet were prepared bythe same process as Example 15, except that a 5-minute heat treatment at230° C. was conducted subsequent to the pressure molding. Results ofmeasurement were substantially the same as those shown in Table 26.

EXAMPLE 18

Sample Nos. 41c to 46c of the rare earth bonded magnet were prepared bythe same process as Example 15, except that a 3-minute heat treatment at200° C. was conducted subsequent to the pressure molding. Results ofmeasurement were substantially the same as those shown in Table 27.

As will be understood from the foregoing description, according to thepresent invention, it is possible to obtain, even with reduced amount ofbinder resin, a rare earth bonded magnet which excels in moldability,porosity, mechanical strength, corrosion resistance (durability),dimensional stability (dimensional precision) and magnetic performance.

An extremely low porosity and further improved dimensional stability areobtainable when the size of the granular material falls within a desiredrange.

The molding may be carried out by warm molding, so that rare earthbonded magnets having advantageous features set forth above can bemanufactured with comparatively low molding pressure, thus offering easeof manufacture. This offers a reduction in the production costs andenhances adaptation to mass-production.

Specifically low porosity and extremely high dimensional stability ofthe rare earth bonded magnet are achieved when the second temperature(pressure releasing temperature) in the cooling step is not higher thanthe melting temperature, in particular not higher than the thermaldeformation temperature, of the thermoplastic resin used, or when thedifference between the first and second temperatures is greater than apredetermined value.

The improvement in the corrosion resistance and dimensional stability,not to mention the magnetic characteristics, greatly contributes toimprovement in the performance of a device such as a micro-motorincorporating the magnet of the invention.

                  TABLE 1                                                         ______________________________________                                                                  Melting  Thermal                                    Resin                     point    deformation                                No.   Binder resin (Thermoplastic resin)                                                                [°C.]                                                                           temp. [°C.]                         ______________________________________                                        A     Polyamide resin (PA12)                                                                            178      145                                        B     Polyamide resin (Copolymer PA6-12)                                                                145      46                                         C     Polyamide resin (PA6)                                                                             215      180                                        D     Polypropylene resin (PP)                                                                          174      105                                        E     Polypropylene resin (PE)                                                                          128      86                                         F     Copolymeric polyester                                                                             280      180                                              (Liquid crystal polymer: LCP)                                           G     Polyphenylene sulfide (PPS)                                                                       287      260                                        ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        (Example 1)                                                                                   Kneading Molding                                                                              Press.  Molding                               Sample Binder   temp.    temp.  release press.                                No.    resin    [°C.]                                                                           [°C.]                                                                         temp. [°C.]                                                                    [kgf/mm.sup.2 ]                       ______________________________________                                        1a     A        150˜250                                                                          220    100     10                                    2a     B        100˜250                                                                          200    40      15                                    3a      A (50%) 150˜250                                                                          230    120     10                                           +B (25%)                                                               4a      A (50%) 140˜250                                                                          210    40      7.5                                          +B (50%)                                                               5a     C        190˜290                                                                          250    150     20                                    6a     D        120˜250                                                                          210    90      25                                    7a     E        100˜200                                                                          150    70      10                                    8a     F        200˜350                                                                          320    140     30                                    9a     G        260˜360                                                                          300    240     25                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        (Example 1)                                                                                                        Mechan-                                  Sam-                            Poro-                                                                              ical   Corrosion                         ple  Br     iHc    (BH) max                                                                             Density                                                                             sity strength                                                                             resistance                        No.  [kG]   [kOe]  [MGOe] [g/cm.sup.3 ]                                                                       [%]  [kgf/mm.sup.2 ]                                                                      [hrs]                             ______________________________________                                        1a   7.21   9.26   10.1   6.01  0.59 7.90   >500                              2a   7.19   9.31   10.0   6.03  0.73 7.45   >500                              3a   7.23   9.23   10.1   6.03  0.37 7.78   >500                              4a   7.22   9.21   10.0   6.03  0.49 7.60   >500                              5a   7.27   9.27   10.0   6.08  0.92 8.10   300                               6a   7.24   9.35   10.3   5.88  0.58 6.95   300                               7a   7.23   9.30   10.2   5.95  0.38 5.80   350                               8a   7.03   9.12   9.8    6.29  0.64 9.65   450                               9a   7.01   9.10   9.8    6.27  0.59 9.73   450                               ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Resin Binder resin   Softening temp.                                                                            Hardening                                   No.   (Themosetting resin)                                                                         [°C.] conditions                                  ______________________________________                                        H     Bisphenol A epoxy resin                                                                      R'm temp. or less                                                                          150° C. 1 hr                                              (Approx. 10 C)                                           I     Phenol novolak resin                                                                         80           170° C. 2 hr                         J     Phenol resin   70           180° C. 4 hr                         ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        (Example 2)                                                                                   Kneading Molding                                                                              Press.  Molding                               Sample Binder   temp.    temp.  release press.                                No.    resin    [°C.]                                                                           [°C.]                                                                         temp. [°C.]                                                                    [kgf/mm.sup.2 ]                       ______________________________________                                        10a    H        R'm temp.                                                                              R'm temp.                                                                            5       60                                    11a    H        R'm temp.                                                                              R'm temp.                                                                            5       65                                    12a    I        80˜100                                                                           120    50      70                                    13a    I        80˜100                                                                           120    50      100                                   14a    J        70˜90                                                                            100    50      100                                   15a    J        70˜90                                                                            100    50      120                                   ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        (Example 2)                                                                                                  Mechanical                                                                            Corrosion                              Sample (BH) max Density Porosity                                                                             strength                                                                              rsistance                              No.    [MGOe]   [g/cm.sup.2 ]                                                                         [%]    [kgf/mm.sup.2 ]                                                                       [hrs]                                  ______________________________________                                        10a    8.8      5.81    5.92   3.75    2000                                   11a    9.1      5.85    5.27   3.86    2000                                   12a    9.0      5.82    5.75   3.91    2000                                   13a    9.3      5.86    5.11   3.98    2000                                   14a    9.2      5.90    6.83   4.01    2000                                   15a    9.4      6.02    4.93   4.11    2000                                   ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        (Example 3)                                                                                        Kneading Kneader                                         Sample               temp.    speed                                           No    Kneader type   [°C.]                                                                           [rpm]  Rate                                     ______________________________________                                        16a   Twin-extrusion kneader                                                                       170˜320                                                                          100˜250                                                                        30 kg/hr                                 17a   Roll kneader   180˜300                                                                           10˜100                                                                        5 kg/batch,                                                                   15 min/batch                             18a   Ordinary kneader                                                                             180˜300                                                                           20˜100                                                                        10 kg/batch,                                                                  30 min/batch                             19a   KCK            170˜320                                                                          20˜80                                                                          20 kg/hr                                 ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        (Example 3)                                                                                                  Mechanical                                                                            Corrosion                              Sample (BH) max Density Porosity                                                                             strength                                                                              resistance                             No.    [MGOe]   [g/cm.sup.3 ]                                                                         [%]    [kgf/mm.sup.2 ]                                                                       [hrs]                                  ______________________________________                                        16a    15.2     6.63    0.65   8.14    >1000                                  17a    15.5     6.65    0.35   8.23    >1000                                  18a    14.9     6.61    0.95   8.09    >1000                                  19a    15.3     6.63    0.65   8.19    >1000                                  ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        (Example 4)                                                                         Mean grain                                                                              Molded product                                                Sample                                                                              size      weight      Density                                                                             Porosity                                                                             Height                               No.   [mm]      [mg]        [g/cm.sup.3 ]                                                                       [%]    [mm]                                 ______________________________________                                        23a   2         1073        5.78  0.30   4.95                                 24a   1.8       1075        5.78  0.30   4.96                                 25a   1.5       1077        5.78  0.30   4.97                                 26a   1         1079        5.78  0.30   4.98                                 27a   0.5       1083        5.79  0.12   4.99                                 28a   0.1       1081        5.79  0.12   4.98                                 29a   0.05      1080        5.76  0.64   5.00                                 30a   0.01      1075        5.72  1.33   5.01                                 31a   0.007     1071        5.68  2.02   5.03                                 ______________________________________                                         Note) Molded product weight is showm in terms of average of 10 products (     = 10)                                                                    

                  TABLE 10                                                        ______________________________________                                        (Example 5)                                                                   Sam-         Molding                     Mechanical                           ple  Binder  temp.    (BH) max                                                                             Density                                                                             Porosity                                                                            strength                             No.  resin   [° C.]                                                                          [MGOe] [g/cm.sup.3 ]                                                                       [%]   [kgf/mm.sup.2 ]                      ______________________________________                                        32a  A       150      17.0   6.21  2.52  5.10                                 33a  A       180      17.5   6.28  1.42  7.10                                 34a  A       200      18.2   6.34  0.48  7.70                                 35a  A       300      17.8   6.32  0.79  7.61                                 36a  A       360      16.2   6.32  0.79  7.55                                 37a  F       190      16.9   6.36  2.70  8.10                                 38a  F       220      17.1   6.40  2.09  8.51                                 39a  F       250      17.5   6.44  1.48  9.25                                 40a  F       300      18.0   6.50  0.56  9.78                                 41a  F       350      17.6   6.50  0.56  9.65                                 42a  F       400      15.5   6.50  0.56  9.60                                 ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        (Example 6)                                                                                 Press.                                                                        release                    Outside                              Sample                                                                              Binder  temp.   (BH) max                                                                             Density                                                                             Porosity                                                                            dimension                            No.   resin   [° C.]                                                                         [MGOe] [g/cm.sup.3 ]                                                                       [%]   [mm]                                 ______________________________________                                        43a   A       180     8.2    6.56  2.51  10.00 ± 0.03                      44a   A       170     8.5    6.62  1.62  10.01 ± 0.03                      45a   A       160     8.6    6.65  1.17  10.00 ± 0.02                      46a   A       140     8.6    6.67  0.87  10.01 ± 0.02                      47a   A       100     8.7    6.68  0.73  10.01 ± 0.01                      48a   G       283     7.7    6.60  3.53  10.03 ± 0.04                      49a   G       275     7.9    6.64  2.95  10.02 ± 0.04                      50a   G       260     8.3    6.74  1.49  10.01 ± 0.03                      51a   G       240     8.5    6.78  0.90  10.00 ± 0.01                      52a   G       200     8.5    6.79  0.76  10.00 ± 0.01                      ______________________________________                                         Note) Outside dimension is showm in terms of average over 10 spesima.    

                  TABLE 12                                                        ______________________________________                                        (Example 7)                                                                        Mold-                                                                    Sam- ing                              Poro-                                                                              Mechanical                         ple  temp.   Br     iHc  (BH) max                                                                             Density                                                                             sity strength                           No.  [° C.]                                                                         [kG]   [kOe]                                                                              [MGOe] [g/cm.sup.3 ]                                                                       [%]  [kgf/mm.sup.2 ]                    ______________________________________                                        1b   170     6.87   9.91 10.2   6.14  3.74 6.97                               2b   175     6.94   9.90 10.6   6.20  2.80 6.99                               3b   180     7.05   9.92 11.4   6.30  1.23 7.11                               4b   200     7.08   9.91 11.5   6.33  0.76 7.53                               5b   220     7.07   9.90 11.5   6.33  0.76 7.51                               6b   240     7.04   9.85 11.2   6.31  1.07 7.58                               ______________________________________                                    

                  TABLE 13                                                        ______________________________________                                                                            Mean                                      Powder                              grain                                     No    Powder type                                                                              Composition        size [μm]                              ______________________________________                                        1     Nd--Fe--B  Nd.sub.12.0 Fe.sub.77.8 Co.sub.4.8 B.sub.5.8                                                     22                                        2     Sm--Co     Sm(Co.sub.0.872 Fe.sub.0.22 Cu.sub.0.08 Zr.sub.0.028).sub                     .8.35              15                                        3     Nanocrystalline                                                                          Nd.sub.5.5 Fe.sub.66 B.sub.10.5 Co.sub.5 Cr.sub.6                                                24                                              Nd--Fe--B                                                               4     Sm--Fe--N  Sm.sub.2 Fe.sub.17 N.sub.3                                                                       2                                         5     HDDR       Ne.sub.12.6 Fe.sub.68.3 Co.sub.12.0 B.sub.6.0 Zr.sub.0.1                                         25                                              Nd--Fe--B                                                               ______________________________________                                    

                  TABLE 14                                                        ______________________________________                                        (Example 8)                                                                                  Press. release                                                 Sample                                                                              Powder   temp.      Density                                                                              Porosity                                                                            Circularity                            No.   No.      [°C.]                                                                             [g/cm.sup.3 ]                                                                        [%]   [μm]                                ______________________________________                                         7b   1        185        5.81   3.97  29.9                                    8b   1        170        5.88   2.81  17.3                                    9b   1        155        5.93   1.98  12.7                                   10b   1        140        6.00   0.82  5.2                                    11b   1        100        6.01   0.66  4.1                                    12b   2        190        6.34   4.22  29.5                                   13b   2        180        6.36   3.91  27.6                                   14b   2        170        6.42   3.00  21.0                                   15b   2        155        6.49   1.95  12.5                                   16b   2        140        6.57   0.74  5.5                                    17b   2        110        6.59   0.43  4.8                                    18b   3        180        5.82   3.80  29.1                                   19b   3        165        5.93   1.98  19.3                                   20b   3        140        5.98   1.16  7.6                                    ______________________________________                                         Continues to Table 15                                                    

                  TABLE 15                                                        ______________________________________                                        (Example 8)                                                                   Sample Powder   Press. release                                                                           Density                                                                              Porosity                                                                            Circularity                           No.    No.      temp. [°C.]                                                                       [g/cm.sup.3 ]                                                                        [%]   [μm]                               ______________________________________                                        21b    3        125        6.00   0.82  5.4                                   22b    3        90         6.02   0.49  4.6                                   23b    2 (75%) +                                                                              182        6.27   3.17  29.1                                         4 (25%)                                                                24b    2 (75%) +                                                                              175        6.29   2.87  23.1                                         4 (25%)                                                                25b    2 (75%) +                                                                              160        6.35   1.98  21.3                                         4 (25%)                                                                26b    2 (75%) +                                                                              135        6.44   0.55  7.4                                          4 (25%)                                                                27b    2 (75%) +                                                                              80         6.46   0.24  3.8                                          4 (25%)                                                                28b    5        175        5.92   2.14  19.7                                  29b    5        150        5.94   1.81  10.1                                  30b    5        135        6.00   0.82  6.1                                   31b    5        100        6.00   0.82  4.0                                   ______________________________________                                    

                                      TABLE 16                                    __________________________________________________________________________                     Melting                                                                             Thermal                                                                             Kneading                                                                           Molding                                     Resin                                                                             Binder resin point deformation                                                                         temp.                                                                              temp.                                       No. (Thermoplastic resin)                                                                      [°C.]                                                                        temp. [°C.]                                                                  [°C.]                                                                       [°C.]                                __________________________________________________________________________    A   Polyamide resin (PA12)                                                                     178   145   150˜250                                                                      230                                         B   Polyamide resin                                                                            145   46    100˜250                                                                      200                                             (Copolymer PA6-12)                                                        C   Polyamide resin (PA6)                                                                      215   180   200˜280                                                                      270                                         D   Polypropylene resin (PP)                                                                   174   105   140˜250                                                                      220                                         E   Polypropylene resin (PE)                                                                   128    86   100˜200                                                                      180                                         F   Liquid crystal polymer (LCP)                                                               280   180   250˜350                                                                      320                                         G   Polyphenylene sulfide (PPS)                                                                287   260   270˜350                                                                      300                                         A + B                                                                             Polyamide resin (mixture)                                                                  162    96   120˜250                                                                      200                                             (PA12:50% + PA6-12:50%)                                                                    (Calculated                                                                         (Calculated                                                             value)                                                                              value)                                                 __________________________________________________________________________

                  TABLE 17                                                        ______________________________________                                        (Example 9)                                                                                    Press.                                                                        release                                                      Sample Resin     temp.    Density                                                                              Porosity                                                                             Circularity                           No.    No.       [°C.]                                                                           [g/cm.sup.3 ]                                                                        [%]    [μm]                               ______________________________________                                        32b    A         185      6.57   4.03   29.6                                  33b    A         170      6.65   2.86   22.1                                  34b    A         140      6.79   0.81   17.8                                  35b    A         110      6.81   0.52   7.7                                   36b    A (50%) + 175      6.65   3.07   38.4                                         B (50%)                                                                37b    A (50%) + 150      6.72   2.05   32.1                                         B (50%)                                                                38b    A (50%) + 135      6.74   1.76   26.7                                         B (50%)                                                                39b    A (50%) +  60      6.79   1.03   7.1                                          B (50%)                                                                40b    A (50%) +  40      6.82   0.59   3.6                                          B (50%)                                                                ______________________________________                                         Continues to Table 18                                                    

                  TABLE 18                                                        ______________________________________                                        (Example 9)                                                                                  Press. release                                                 Sample Resin   temp.      Density                                                                              Porosity                                                                            Circularity                            No.    No.     [°C.]                                                                             [g/cm.sup.3 ]                                                                        [%]   [μm]                                ______________________________________                                        41b    C       215        6.68   4.00  26.6                                   42b    C       210        6.70   3.72  21.1                                   43b    C       190        6.74   3.14  15.5                                   44b    C       170        6.80   2.28  9.6                                    45b    C       125        6.92   0.56  6.4                                    46b    D       180        6.47   3.54  28.9                                   47b    D       165        6.52   2.80  20.9                                   48b    D       115        6.57   2.05  16.6                                   49b    D        95        6.68   0.41  7.8                                    50b    E       125        6.53   3.53  19.6                                   51b    E       110        6.57   2.93  15.3                                   ______________________________________                                         Continues to Table 19                                                    

                  TABLE 19                                                        ______________________________________                                        (Example 9)                                                                   Sample Resin   Press. release                                                                           Density                                                                              Porosity                                                                            Circularity                            No.    No.     temp. [°C.]                                                                       [g/cm.sup.3 ]                                                                        [%]   [μm]                                ______________________________________                                        52b    E       95         6.63   2.05  12.1                                   53b    E       78         6.70   1.01  9.6                                    54b    F       290        6.90   3.43  36.6                                   55b    F       275        6.95   2.73  18.6                                   56b    F       250        6.98   2.31  15.3                                   57b    F       190        7.08   0.91  7.1                                    58b    F       170        7.11   0.49  3.6                                    59b    G       292        6.86   3.66  30.0                                   60b    G       280        7.05   0.99  7.2                                    61b    G       255        7.09   0.43  3.8                                    62b    G       200        7.09   0.43  3.4                                    ______________________________________                                    

                  TABLE 20                                                        ______________________________________                                                 Magnetic                                                                      powder   Binder resin                                                                            Antioxidant                                                                           Other additive                            Composition                                                                            [wt %]   [wt %]    [wt %]  [wt %]                                    ______________________________________                                        Composition 1                                                                          92.0     7.1       0.7     Silicone oil                                                                  0.2                                       Composition 2                                                                          94.0     4.7       1.2     Oleic acid                                                                    0.1                                       Composition 3                                                                          96.0     2.4       1.6     --                                        Composition 4                                                                          98.0     1.0       1.0     --                                        ______________________________________                                    

                  TABLE 21                                                        ______________________________________                                        (Example 10)                                                                                  Press.                                                        Sam-            release             Circu-                                    ple             temp.   Density                                                                             Porosity                                                                            larity                                                                              (BH) max                            No.  Composition                                                                              [° C.]                                                                         [g/cm.sup.3 ]                                                                       [%]   [μm]                                                                             [MGOe]                              ______________________________________                                        63b  Composition 1                                                                            178     4.81  3.97  29.4  6.0                                 64b  Composition 1                                                                            170     4.83  3.58  27.1  6.1                                 65b  Composition 1                                                                            140     4.86  2.98  22.1  6.1                                 66b  Composition 1                                                                            100     4.99  0.38  4.1   6.2                                 67b  Composition 2                                                                            178     5.27  3.79  27.9  7.0                                 68b  Composition 2                                                                            170     5.30  3.24  24.5  7.0                                 69b  Composition 2                                                                            140     5.35  2.33  17.1  7.1                                 70b  Composition 2                                                                            100     5.45  0.50  4.4   7.1                                 ______________________________________                                         Continues to Table 22                                                    

                  TABLE 22                                                        ______________________________________                                        (Example 10)                                                                                  Press.                                                        Sam-            release             Circu-                                    ple             temp.   Density                                                                             Porosity                                                                            larity                                                                              (BH) max                            No.  Composition                                                                              [° C.]                                                                         [g/cm.sup.3 ]                                                                       [%]   [μm]                                                                             [MGOe]                              ______________________________________                                        71b  Composition 3                                                                            180     5.82  3.91  28.9  9.2                                 72b  Composition 3                                                                            178     5.87  3.09  23.8  9.4                                 73b  Composition 3                                                                            170     5.89  2.76  17.8  9.6                                 74b  Composition 3                                                                            140     6.01  0.78  5.5   10.0                                75b  Composition 3                                                                            100     6.02  0.61  4.3   10.1                                76b  Composition 4                                                                            185     6.52  3.10  27.6  10.5                                77b  Composition 4                                                                            178     6.57  2.36  19.7  10.6                                78b  Composition 4                                                                            170     6.59  2.02  14.0  10.9                                79b  Composition4                                                                             140     6.62  1.62  5.1   11.2                                80b  Composition 4                                                                            100     6.66  1.02  4.8   11.4                                ______________________________________                                    

                                      TABLE 23                                    __________________________________________________________________________           Magnetic powder                                                                            Resin            Additive                                 __________________________________________________________________________    Composition 1                                                                        Nd--Fe--B    Polyamide resin  Hydrazine type antioxidant                      (94.0 wt %)  (No. A in Table 1)                                                                             (1.2 wt %)                                                   (4.8 wt %)                                                Composition 2                                                                        Nd--Fe--B    Polyamide resin  Hydrazine type antioxidant                      (97.0 wt %)  (No. A in Table 1)                                                                             (1.5 wt %)                                                   (1.5 wt %)                                                Composition 3                                                                        Sm--Co       PPS resin        Stearic acid                                    (92.5 wt %)  (No. G in Table i)                                                                             (0.05 wt %)                                                  (7.45 wt %)                                               Composition 4                                                                        Sm--Co +     Polypropylene resin                                                                            Silicone oil                                    Sm--Fe--N    (No. D in Table 1)                                                                             (0.1 wt %)                                      (71 + 24 wt %)                                                                             (4.9 wt %)                                                Composition 5                                                                        Nanocrystalline                                                                            Polyamide resin  Hydrazine type antioxidant                      Nd--Fe--B    (PA11: Thermal deform temp. 150 C.)                                                            (1.0 wt %)                                      (96.0 wt %)  (3.0 wt %)                                                Composition 6                                                                        Nd--Fe--B    Liquid crystal polymer                                                                         Phenol type antioxidant                         (HDDR process) (95.5 wt %)                                                                 (No. F in Table 1)                                                                             (1.2 wt %)                                                   (3.3 wt %)                                                __________________________________________________________________________    Nd--Fe--B:   Nd.sub.12.0 Fe.sub.77.8 Co.sub.4.8 B.sub.5.8                                                 mean grain size 15 μm                          Sm--Co:      Sm(Co.sub.0.872 Fe.sub.0.22 Cu.sub.0.08 Zr.sub.0.028).sub.8.3                 5              mean grain size 18 μm                          Sm--Fe--N:   Sm.sub.2 Fe.sub.17 N.sub.3                                                                   mean grain size 2 μm                           Nanocrystalline Nd--Fe--B:                                                                 Nd.sub.5.5 Fe.sub.66 B.sub.10.5 Co.sub.5 Cr.sub.6                                            mean grain size 20 μm                          Nd--Fe--B (HDDR process):                                                                  Ne.sub.12.6 Fe.sub.68.3 Co.sub.12.0 B.sub.6.0 Zr.sub.0.1                                     mean grain size 20 μm                      

                  TABLE 24                                                        ______________________________________                                        (Example 11)                                                                                 Mold-                                                          Sam-           ing                            Poro-                           ple            temp.   Br   iHc  (BH) max                                                                             Density                                                                             sity                            No.  Material  [° C.]                                                                         [kG] [kOe]                                                                              [MGOe] [g/cm.sup.3 ]                                                                       [%]                             ______________________________________                                        1c   Composition                                                                             210     5.88 9.09 6.9    5.44  0.79                                 1                                                                        2c   Composition                                                                             230     7.48 9.32 11.2   6.33  0.81                                 2                                                                        3c   Composition                                                                             300     6.48 9.18 9.2    5.98  1.81                                 3                                                                        4c   Composition                                                                             200     9.05 12.02                                                                              17.5   6.54  0.94                                 4                                                                        5c   Composition                                                                             220     7.43 4.47 7.7    5.81  0.95                                 5                                                                        6c   Composition                                                                             290     8.55 13.7 15.9   6.12  1.10                                 6                                                                        ______________________________________                                         Sample Nos. 1c, 2c, 5c: No magnetic field applied.                            Sample Nos. 3c, 4c, 6c: Radial magnetic field of 15 kOe applied.         

                  TABLE 25                                                        ______________________________________                                        (Comparison Ex. 3)                                                                                            Press.                                        Sam-            Molding  Molding                                                                              release     Poro-                             ple             temp.    press. temp  Density                                                                             sity                              No.  Material   [° C.]                                                                          [kgf/mm.sup.2 ]                                                                      [C.]  [g/cm.sup.3 ]                                                                       [%]                               ______________________________________                                         7c  Composition 1                                                                            100      80     50    4.98  9.18                               8c  Composition 2                                                                            120      80     50    5.70  10.68                              9c  Composition 3                                                                            235      80     100   5.52  9.37                              10c  Composition 4                                                                            90       80     30    5.35  8.79                              11c  Composition 5                                                                            80       80     50    5.41  10.42                             12c  Composition 6                                                                            150      80     60    5.44  12.10                             ______________________________________                                         Sample Nos. 7c, 8c, 11c: No magnetic field applied.                           Sample Nos. 9c, 10c, 12c: Tadial magnetic field of 15 kOe applied.       

                  TABLE 26                                                        ______________________________________                                        (Example 15)                                                                        Max.              Molded                                                      grain   Mean grain                                                                              product      Molded product                           Sample                                                                              size    size      density                                                                              Porosity                                                                            height                                   NO.   [mm]    [mm]      [g/cm.sup.3 ]                                                                        [%]   [mm]                                     ______________________________________                                        21c   2.0     1.5       6.42   0.42  9.95                                     22c   1.0     0.8       6.42   0.42  9.99                                     23c   0.5     0.45      6.41   0.58  10.00                                    24c   0.3     0.2       6.40   0.73  10.00                                    25c   0.1     0.05      6.37   1.20  10.02                                    26c   0.05    0.04      6.30   2.28  10.04                                    27c   0.02    0.01      6.26   2.50  10.05                                    ______________________________________                                    

                  TABLE 27                                                        ______________________________________                                        (Examle 16)                                                                                      Weight of   Molded                                                                              Molded                                                      granular    product                                                                             product                                  Sample  "a" dimension                                                                            material    weight                                                                              length                                   No.     [mm]       [g]         [g]   [mm]                                     ______________________________________                                        28c     4.0        2.53        2.31  9.12                                     29c     4.5        2.85        2.66  9.35                                     30c     4.8        3.04        3.01  9.90                                     31c     5.0        3.17        3.15  9.95                                     32c     5.5        3.48        3.48  10.00                                    33c     6.0        3.8         3.79  9.99                                     ______________________________________                                    

Industrial Applicability

By virtue of the advantages described hereinbefore, the presentinvention finds extensive use such as, for example, use as permanentmagnets of various motors, e.g., stepping motors, brushless motors andso forth, permanent magnets of solenoids and actuators, permanentmagnets of automotive sensors, permanent magnets of finders of VTRs orthe like, and permanent magnets incorporated in various types of meters.

We claim:
 1. A method for manufacturing a rare earth bonded magnet formed by binding a rare earth magnetic powder by a binder resin, comprising the steps of:mixing said magnet powder and said binder resin and kneading the mixture at a temperature above a thermal deformation temperature of said binder resin so as to prepare a kneaded material; granulating or graining the kneaded material to form the kneaded material into a granular material; conducting a pressure molding on said granulated material at a first temperature more than 20° C. greater than a melting temperature of said binder resin at which said binder resin is softened or molten; and cooling the molded body while keeping said molded body under a constant pressure at least until said molded body is cooled down to a second temperature which is equal to or below said melting temperature of said binder resin.
 2. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein said binder resin is a thermoplastic resin.
 3. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein the content of said rare earth magnet powder in said kneaded material ranges from 90 wt % to 99 wt %.
 4. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein said kneaded material contains an antioxidant.
 5. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein the average granule size of said granular material ranges from 0.01 to 2 mm.
 6. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein said second temperature the thermal deformation temperature of said binder resin.
 7. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein the pressure applied to said molded body during cooling under pressure is maintained constant at a first pressure until the temperature falls to said melting temperature of said binder resin and then said pressure is reduced to a second pressure until the temperature falls to said thermal deformation temperature of said binder resin.
 8. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein the maximum granule size of said granular material is not greater than the minimum size of the gap in the mold used for the molding.
 9. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein the maximum granule size of said granular material is not smaller than 0.02 mm.
 10. A method of manufacturing a rare earth bonded magnet according to claim 1, wherein the granulation or the graining is conducted by grinding.
 11. A method of manufacturing a rare earth bonded magnet according to claim 1, further comprising the step of conducting a heat treatment subsequent to the molding.
 12. A method of manufacturing a rare earth bonded magnet formed by binding a rare earth magnet powder by a thermoplastic binder resin, comprising the steps of:conduction-molding a composition containing said rare earth magnet powder and said binding resin at a first temperature at least 50° C. greater than a melting temperature of said binder resin at which said binder resin is softened or molten; and cooling the molded body while keeping said molded body under pressure until said molded body is cooled down to a thermal deformation temperature of said binder resin which is below said first temperature.
 13. A method of manufacturing a rare earth bonded magnet according to claim 12, wherein said second temperature is the melting temperature or the thermal deformation temperature of said binder resin.
 14. A method of manufacturing a rare earth bonded magnet according to claim 12, wherein the cooling under pressure is conducted continuously without releasing the pressure applied during the pressure molding.
 15. A method of manufacturing a rare earth bonded magnet according to claim 12, wherein the pressure applied during the cooling under pressure is equal to or lower than the pressure applied during the pressure molding.
 16. A method of manufacturing a rare earth bonded magnet according to claim 12, wherein the pressure applied during the cooling under pressure is maintained constant at least in the period in which the temperature comes down to the second temperature of said binder resin.
 17. A method of manufacturing a rare earth bonded magnet according to claim 12, wherein the rate of cooling under pressure ranges from 0.5° C./sec to 100° C./sec.
 18. A method of manufacturing a rare earth bonded magnet according to claim 12, wherein the pressure applied during the pressure molding is not higher than 60 kgf/mm².
 19. The method of claim 12, wherein the content of said rare earth magnet powder in said magnet ranges from 92.0 wt % to 99.0 wt %.
 20. The method of claim 12, wherein said magnet molded in the absence of magnetic field exhibits maximum magnetic energy product (BH)max of not lower than 6 MGOe.
 21. The method of claim 12, wherein said magnet molded under the influence of a magnetic fieid exhibits maximum magnetic energy product (BH)max of not lower than 12 MGOe.
 22. A method of manufacturing a rare earth bonded magnet formed by binding a rare earth magnet powder by a thermoplastic binder resin, comprising the steps of:kneading a composition containing said rare earth magnet powder and said binding resin at a temperature which is not lower than the thermal deformation temperature of said binder resin; pressure-molding the kneaded material at a first temperature at which said binder resin is softened or molten; and cooling the molded body while keeping said molded body under a first pressure until said molded body is cooled down to a second temperature which is below said first temperature and further cooling the molded body while keeping said molded body under a second pressure which is about 50-80% of said first pressure.
 23. A method of manufacturing a rare earth bonded magnet according to claim 22, wherein said second temperature is the melting temperature or the thermal deformation temperature of said binder resin.
 24. A method of manufacturing a rare earth bonded magnet according to claim 22, wherein the difference between said first and second temperatures is not smaller than 20° C.
 25. A method of manufacturing a rare earth bonded magnet according to claim 22, wherein the cooling under pressure is conducted continuously without releasing the pressure applied during the pressure molding.
 26. A method of manufacturing a rare earth bonded magnet according to claim 22, wherein the pressure applied during the cooling under pressure is equal to or lower than the pressure applied during the pressure molding.
 27. A method of manufacturing a rare earth bonded magnet according to claim 22, wherein the pressure applied during the cooling under pressure is maintained constant at least in the period in which the temperature comes down to the second temperature.
 28. A method of manufacturing a rare earth bonded magnet according to claim 22, wherein the rate of cooling under pressure ranges from 0.5° C./sec to 100° C./sec.
 29. A method of manufacturing a rare earth bonded magnet according to claim 23, wherein the pressure applied during the pressure molding is not higher than 60 kgf/mm².
 30. The method of claim 22, wherein the content of said rare earth magnet powder in said magnet ranges from 92.0 wt % to 99.0 wt %.
 31. The method of claim 22, wherein said magnet molded in the absence of magnetic field exhibits maximum magnetic energy product (BH)max of not lower than 6 MGOe.
 32. The method of claim 22, wherein said magnet molded under the influence of a magnetic field exhibits maximum magnetic energy product (BH)max of not lower than 12 MGOe. 